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/Button.cpp

@@ -35,8 +35,8 @@
 
 namespace GUI {
 
-Button::Button(const StringView& text)
-    : AbstractButton(text)
+Button::Button(String text)
+    : AbstractButton(move(text))
 {
     set_focus_policy(GUI::FocusPolicy::StrongFocus);
 }