Page in PagedTileLayout only changes when needed

Made sure that pages are not changed unnecesarily on events that do not
modify the page structure.

Saves current page on destruction, to be restored later. In particular
in the case of switching to Dark theme on entering battery saver mode
(it destroys QSFragment).

Test: manual && runtest
Change-Id: I941f0a7728139257d5c5dd3646df16aaf1805470
Fixes: 117171669