With Qt 5.15.2 (and clang), `k->first` works, but this breaks
with Qt 5.11 (and gcc), this is not available and the dereference
must be written differently, `(*k).first`.
- This kind of runs around the selection model on the view,
but we're drawing radio buttons ourselves **anyway**
and the list of themes knows which is selected / current
independent of the view.
- make ThemeInfo and ThemeInfoList internal, expose only
ThemesModel to the rest of the PlasmaLnF module
- don't build the widget anymore (needs to be replaced by
a delegate)
- also individual changes need to be signalled
- use QSignalBlocker to avoid spamming changes when calling
aggregate change methods
- refactor findById() so that also a row number can be
obtained, which is needed for the change signals.
- put a filter model in place, so only the themes with "show" set
are displayed
- rip out the messing about with widgets, soon to introduce a model-
based UI
