GUI Buttons should accept pure touch events

[Foxysen]

I am toying with an RTS game, which I want to be playable both on desktop and touch screen. Despite the similarity between mouse and touch input, they feel different enough that I want to implement different ways to control. In order to do that, I disable emulation of mouse from touch screen.

The problem that as soon as I do that, all of GUI menus become useless and unpassable. There is touchscreen buttons nodes, but they aren't designed to work as GUI, seems more like quick and neat virtual keyboard style buttons for gameplay.

Letting GUI buttons accept touch input would be good because otherwise in combined mode (emulation of mouse from touch screen) I have to do extra work to ignore mouse events in gameplay by detecting whether system uses touch screen. Which won't be pretty for Android users with a mouse.

[Calinou]

Thanks for the proposal! Consolidating in #3976.