gtktiledviewer: allow findbar in multiple windows

With this, all previously global data is now stored per-window in the
g_aWindows map.

(cherry picked from commit d51e3f78)

Conflicts:
	libreofficekit/qa/gtktiledviewer/gtktiledviewer.cxx

Change-Id: I45b4449ee7d516106ea0b039d1af97db49edf759