ApplicationListSearchModel: beginFilterChange() was called after
mutating m_categoryFilter, inverting the documented before/after
contract. Also add null checks for sourceModel() and
favouritesModel() in filterAcceptsRow.
CategoryPanel: add onModelReset to repopulate the category list
when the underlying model is fully reloaded, not just on individual
row insertions and removals.
A panel between the drawer and the power panel lists the XDG
categories installed on the device as icon-and-label tiles.
Tapping one filters the app list; text search still applies
on top. The filter resets each time the drawer opens. Audio,
Video and Settings fold into their parent groups to match
Kickoff.
