Add central Widget struct + clean up code

In particular, move all termbox2 usage to TerminalBuffer.zig &
keyboard.zig

Signed-off-by: AnErrupTion <anerruption@disroot.org>

src/tui/Widget.zig

@@ -0,0 +1,150 @@
+const Widget = @This();
+
+const keyboard = @import("keyboard.zig");
+const TerminalBuffer = @import("TerminalBuffer.zig");
+
+const VTable = struct {
+    deinit_fn: *const fn (ptr: *anyopaque) void,
+    realloc_fn: *const fn (ptr: *anyopaque) anyerror!void,
+    draw_fn: *const fn (ptr: *anyopaque) void,
+    update_fn: *const fn (ptr: *anyopaque, ctx: *anyopaque) anyerror!void,
+    handle_fn: *const fn (ptr: *anyopaque, maybe_key: ?keyboard.Key, insert_mode: bool) anyerror!void,
+};
+
+pointer: *anyopaque,
+vtable: VTable,
+
+pub fn init(
+    pointer: anytype,
+    comptime deinit_fn: ?fn (ptr: @TypeOf(pointer)) void,
+    comptime realloc_fn: ?fn (ptr: @TypeOf(pointer)) anyerror!void,
+    comptime draw_fn: ?fn (ptr: @TypeOf(pointer)) void,
+    comptime update_fn: ?fn (ptr: @TypeOf(pointer), ctx: *anyopaque) anyerror!void,
+    comptime handle_fn: ?fn (ptr: @TypeOf(pointer), maybe_key: ?keyboard.Key, insert_mode: bool) anyerror!void,
+) Widget {
+    const Pointer = @TypeOf(pointer);
+    const Impl = struct {
+        pub fn deinitImpl(ptr: *anyopaque) void {
+            const impl: Pointer = @ptrCast(@alignCast(ptr));
+
+            if (deinit_fn) |func| {
+                return @call(
+                    .always_inline,
+                    func,
+                    .{impl},
+                );
+            }
+        }
+
+        pub fn reallocImpl(ptr: *anyopaque) !void {
+            const impl: Pointer = @ptrCast(@alignCast(ptr));
+
+            if (realloc_fn) |func| {
+                return @call(
+                    .always_inline,
+                    func,
+                    .{impl},
+                );
+            }
+        }
+
+        pub fn drawImpl(ptr: *anyopaque) void {
+            const impl: Pointer = @ptrCast(@alignCast(ptr));
+
+            if (draw_fn) |func| {
+                return @call(
+                    .always_inline,
+                    func,
+                    .{impl},
+                );
+            }
+        }
+
+        pub fn updateImpl(ptr: *anyopaque, ctx: *anyopaque) !void {
+            const impl: Pointer = @ptrCast(@alignCast(ptr));
+
+            if (update_fn) |func| {
+                return @call(
+                    .always_inline,
+                    func,
+                    .{ impl, ctx },
+                );
+            }
+        }
+
+        pub fn handleImpl(ptr: *anyopaque, maybe_key: ?keyboard.Key, insert_mode: bool) !void {
+            const impl: Pointer = @ptrCast(@alignCast(ptr));
+
+            if (handle_fn) |func| {
+                return @call(
+                    .always_inline,
+                    func,
+                    .{ impl, maybe_key, insert_mode },
+                );
+            }
+        }
+
+        const vtable = VTable{
+            .deinit_fn = deinitImpl,
+            .realloc_fn = reallocImpl,
+            .draw_fn = drawImpl,
+            .update_fn = updateImpl,
+            .handle_fn = handleImpl,
+        };
+    };
+
+    return .{
+        .pointer = pointer,
+        .vtable = Impl.vtable,
+    };
+}
+
+pub fn deinit(self: *Widget) void {
+    const impl: @TypeOf(self.pointer) = @ptrCast(@alignCast(self.pointer));
+
+    return @call(
+        .auto,
+        self.vtable.deinit_fn,
+        .{impl},
+    );
+}
+
+pub fn realloc(self: *Widget) !void {
+    const impl: @TypeOf(self.pointer) = @ptrCast(@alignCast(self.pointer));
+
+    return @call(
+        .auto,
+        self.vtable.realloc_fn,
+        .{impl},
+    );
+}
+
+pub fn draw(self: *Widget) void {
+    const impl: @TypeOf(self.pointer) = @ptrCast(@alignCast(self.pointer));
+
+    return @call(
+        .auto,
+        self.vtable.draw_fn,
+        .{impl},
+    );
+}
+
+pub fn update(self: *Widget, ctx: *anyopaque) !void {
+    const impl: @TypeOf(self.pointer) = @ptrCast(@alignCast(self.pointer));
+
+    return @call(
+        .auto,
+        self.vtable.update_fn,
+        .{ impl, ctx },
+    );
+}
+
+pub fn handle(self: *Widget, maybe_key: ?keyboard.Key, insert_mode: bool) !void {
+    const impl: @TypeOf(self.pointer) = @ptrCast(@alignCast(self.pointer));
+
+    return @call(
+        .auto,
+        self.vtable.handle_fn,
+        .{ impl, maybe_key, insert_mode },
+    );
+}