DictionaryComboBox: Keep user preferred dictionaries on top

Summary:
...to ease the switch between dictionaries you usually need

CCBUG: 302689

Test Plan:
Three shots how it looks in Kate

Normal size

Expaned to showcase the included splitter...

...and how the combobox looks

Reviewers: Frameworks, VDG, ngraham, cullmann

Reviewed By: VDG, ngraham, cullmann

Subscribers: cullmann, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17030