Make alphabetic categorization of the "All Applications" section optional

Summary:
Introduces a new prop and turns it off by default.

Also rewrites the initial "all applications" data gathering avoid
a crash uncovered while working on this (it didn't check EntryType)
and reduces assumptions about the model topology.

It also makes it collect root-level apps now, which was forgotten
to be added when support for root-level apps were added.

This supercedes D22762 and D22890. It does less duplicate work and fixes
more bugs while also not breaking the Dashboard UI.

Reviewers: Plasma, davidedmundson, tcanabrava

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22901