windowManager: Use own variables for each gesture

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.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/598