kcms/lnf: Simplify Theme's contents and selection representation

Currently the code to represent a Theme's contents and the user selection use different representations at different places which make it a bit tangled and less scalable, requiring some boilerplate both in C++ and QML to map between them:

• Theme's contents: they are read in KCMLookAndFeel and stored as multiple boolean roles in the themes' model.
• User selection: they are stored as two different enum flags (for appearance and layout) in the LookAndFeelManager

This MR tries to improve this situation:

• use a common representation (Contents enum flags)
• move the user selection to the KCMLookAndFeel class.
• move the theme management bits (which deals with the package files) together in the LookAndFeelManager
• move and adapt also the "remove theme" functionality added in !2499 (merged)
• reduce the amount of model roles required

Presented as a series of atomic patches to hopefully ease review and in case of unexpected regressions, but they can as well get squashed.

I'm not sure whether to target Plasma 6 or 5.27 due to the soft freeze, but I wonder if that will make it more difficult in the future to port back possible bugfixes.