优化布局语法，消除部分警告

example/src/main.cpp

@@ -68,31 +68,28 @@ int main(int argc, char* argv[]) {
 	const auto& text_block2 = std::make_shared<mtext_block>();
 	text_block2->set_text(U"Hello, World!");
 
-	auto button = mnew(mbutton)
-	[
-		mslot(mbutton)
-		.margin({ 10 })
-		.visibility(visibility_t::visible)(text_block)
-	];
-
-	auto button2 = mnew(mbutton)
-	[
-		mslot(mbutton)
-		.margin({ 10 })
-		.visibility(visibility_t::visible)(text_block2)
-	];
-
 	const auto& window = mwindow::create({ 800, 600 }, L"Hello, World!");
 	window->set_content(
 		mnew(mv_box)
 		[
 			mslot(mv_box)
 			.horizontal_alignment(horizontal_alignment_t::left)
-			(button),
+			+mnew(mbutton)
+			[
+				mslot(mbutton)
+				.margin({ 10 })
+				.visibility(visibility_t::visible)(text_block)
+			],
 
 			mslot(mv_box)
 			.horizontal_alignment(horizontal_alignment_t::right)
-			(button2)
+			+mnew(mbutton)
+			[
+				mslot(mbutton)
+				.margin({ 10 })
+				.visibility(visibility_t::visible)
+				(text_block2)
+			]
 		]
 	);