embed, view-container: Let each mode have its own no-results page

... and do not hide it on PhotosTrackerChangeMonitor::changes-pending.
Only hide it when the count of the mode-specific
PhotosOffsetController goes above zero.

Fixes: https://bugzilla.gnome.org/720570
embed, view-container: Let each mode have its own error page

Fixes: https://bugzilla.gnome.org/720570
empty-results-box: Tailor it to the specific modes
searchbar: Copy the widget's internal string

... so that it does not change underneath us.
embed: Fix the interactions between search, collection view and preview

We might want to have a separate mode for collection view in future.
Prepare 3.11.5