add(examples/demo): application to showcase a more complex application

Further improvements for example applications; Demo example is now
default build target (when not providing example configuration).

examples/elements/button.zig

@@ -52,9 +52,7 @@ pub fn main() !void {
     errdefer |err| log.err("Application Error: {any}", .{err});
 
     var gpa: std.heap.GeneralPurposeAllocator(.{}) = .init;
-    defer if (gpa.deinit() == .leak) {
-        log.err("memory leak", .{});
-    };
+    defer if (gpa.deinit() == .leak) log.err("memory leak", .{});
 
     const allocator = gpa.allocator();
 

examples/elements/input.zig

@@ -72,9 +72,7 @@ pub fn main() !void {
     errdefer |err| log.err("Application Error: {any}", .{err});
 
     var gpa: std.heap.GeneralPurposeAllocator(.{}) = .init;
-    defer if (gpa.deinit() == .leak) {
-        log.err("memory leak", .{});
-    };
+    defer if (gpa.deinit() == .leak) log.err("memory leak", .{});
 
     const allocator = gpa.allocator();