add(element/radio-button): RadioButton Element implementation

This can be used to visualize the values of `bool`'s, which is relevant
when creating form's based on `struct`'s automatically.

examples/elements/radio-button.zig

@@ -0,0 +1,98 @@
+const QuitText = struct {
+    const text = "Press ctrl+c to quit.";
+
+    pub fn element(this: *@This()) App.Element {
+        return .{ .ptr = this, .vtable = &.{ .content = content } };
+    }
+
+    fn content(ctx: *anyopaque, cells: []zterm.Cell, size: zterm.Point) !void {
+        _ = ctx;
+        assert(cells.len == @as(usize, size.x) * @as(usize, size.y));
+
+        const row = 2;
+        const col = size.x / 2 -| (text.len / 2);
+        const anchor = (row * size.x) + col;
+
+        for (text, 0..) |cp, idx| {
+            cells[anchor + idx].style.fg = .white;
+            cells[anchor + idx].style.bg = .black;
+            cells[anchor + idx].cp = cp;
+
+            // NOTE do not write over the contents of this `Container`'s `Size`
+            if (anchor + idx == cells.len - 1) break;
+        }
+    }
+};
+
+pub fn main() !void {
+    errdefer |err| log.err("Application Error: {any}", .{err});
+
+    var gpa: std.heap.DebugAllocator(.{}) = .init;
+    defer if (gpa.deinit() == .leak) log.err("memory leak", .{});
+
+    const allocator = gpa.allocator();
+
+    var app: App = .init;
+    var renderer = zterm.Renderer.Buffered.init(allocator);
+    defer renderer.deinit();
+
+    var radiobutton: App.RadioButton = .init(false, .{
+        .label = "Test Radio Button",
+        .style = .squared,
+    });
+    var button: App.Button(.accept) = .init(&app.queue, .init(.default, "Button"));
+    var quit_text: QuitText = .{};
+
+    var container = try App.Container.init(allocator, .{
+        .rectangle = .{ .fill = .grey },
+        .layout = .{ .padding = .all(5) },
+    }, quit_text.element());
+    defer container.deinit();
+    try container.append(try .init(allocator, .{}, radiobutton.element()));
+    try container.append(try App.Container.init(allocator, .{ .rectangle = .{ .fill = .black } }, button.element()));
+
+    try app.start();
+    defer app.stop() catch |err| log.err("Failed to stop application: {any}", .{err});
+
+    // event loop
+    while (true) {
+        const event = app.nextEvent();
+        log.debug("received event: {s}", .{@tagName(event)});
+
+        // pre event handling
+        switch (event) {
+            .key => |key| if (key.eql(.{ .cp = 'c', .mod = .{ .ctrl = true } })) app.quit(),
+            .accept => log.info("Clicked built-in button using the mouse", .{}),
+            .err => |err| log.err("Received {s} with message: {s}", .{ @errorName(err.err), err.msg }),
+            else => {},
+        }
+
+        container.handle(event) catch |err| app.postEvent(.{
+            .err = .{
+                .err = err,
+                .msg = "Container Event handling failed",
+            },
+        });
+
+        // post event handling
+        switch (event) {
+            .quit => break,
+            else => {},
+        }
+
+        container.resize(try renderer.resize());
+        container.reposition(.{});
+        try renderer.render(@TypeOf(container), &container);
+        try renderer.flush();
+    }
+}
+
+pub const panic = App.panic_handler;
+const log = std.log.scoped(.default);
+
+const std = @import("std");
+const assert = std.debug.assert;
+const zterm = @import("zterm");
+const App = zterm.App(union(enum) {
+    accept,
+});