add(example): WIP popup Element example implementation

build.zig

@@ -10,6 +10,7 @@ pub fn build(b: *std.Build) void {
         alignment,
         button,
         input,
+        popup,
         progress,
         scrollable,
         // layouts:
@@ -61,6 +62,7 @@ pub fn build(b: *std.Build) void {
                 .alignment => "examples/elements/alignment.zig",
                 .button => "examples/elements/button.zig",
                 .input => "examples/elements/input.zig",
+                .popup => "examples/elements/popup.zig",
                 .progress => "examples/elements/progress.zig",
                 .scrollable => "examples/elements/scrollable.zig",
                 // layouts: