LibGUI: Update buttons' text/tooltips in Action::set_text

This allows Actions that change based on some state to update their
associated buttons' text and tooltips to match their new text. The
play/pause button in SoundPlayer (and VideoPlayer if it's merged)
will now change tooltips when the playback changes state, rather than
always displaying "Play (Space)".

Userland/Libraries/LibGUI/Button.h

@@ -50,6 +50,7 @@ public:
     Action* action() { return m_action; }
     Action const* action() const { return m_action; }
     void set_action(Action&);
+    void set_text_from_action();
 
     virtual bool is_uncheckable() const override;