Remove entries from favorites when desktop file was deleted

When the KSycoca database changes, we re-evaluate if our app still exists and update the internal service.
The model checks the validity of each of the entires and removes ones that became invalid.

Based on my testing it seemed like one had to type any query to make sure that the databaseChanged signal is emitted.

Test Plan:
`printf "[Desktop Entry]\nIcon=firefox\nName=DelmeFirefox\nExec=notify-send bla bla" >~/.local/share/applications/delmefirefox.desktop`
Add the entry to the favourites
`rm ~/.local/share/applications/delmefirefox.desktop`
Make sure it does not appear in the search
Make sure the favorite is removed

BUG: 393380
FIXED-IN: 5.26