Rename a few functions
Simplify search

With the search entry now being only in the overview, we are
always going back to the overview when exiting search, so
no need to keep a search_back_id.
Improve back button handling

Add a stack, so we can go back from an application to the category
to the overview.
Show installed apps in the main window

Replace the popup dialog we had here with going to the
details page. The improved back button handling lets us
go back to the installed page from there.