LibGUI: Tweak AbstractButton and subclass constructors

Taking a "const StringView&" for the initial text does not achieve
anything useful. Just take a "String" and move it into storage.

Libraries/LibGUI/RadioButton.cpp

@@ -33,8 +33,8 @@
 
 namespace GUI {
 
-RadioButton::RadioButton(const StringView& text)
-    : AbstractButton(text)
+RadioButton::RadioButton(String text)
+    : AbstractButton(move(text))
 {
 }