Fix button up and down events with focus changes

Adds a flag to guard button_up and button_down events based on whether
button_down has been previously emitted. Buttons now emit button_up
signals if they have emitted button_down and subsequently lose focus,
do not emit button_up if they gain focus while ui_accept is still
pressed, and do not emit multiple up/down signals if multiple
ui_accept keys are pressed simultaneously.

scene/gui/base_button.h

@@ -61,7 +61,7 @@ private:
 		bool hovering = false;
 		bool press_attempt = false;
 		bool pressing_inside = false;
-
+		bool pressed_down_with_focus = false;
 		bool disabled = false;
 
 	} status;