windowManager: Use own variables for each gesture

Using one variable to initialize all gestures will update the address of the "gesture" pointer with every newly initialized object. This means that event handlers which also use the "gesture" pointer like the 'keyboard-visible-changed' handler will update a different gesture as soon as the pointer is changed.

This lead to a bug where the handler of 'keyboard-visible-changed' wrongly nabled the unfullscreen gesture. Fix that by assigning each gesture its own variable.