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
{F6442592}

Expaned to showcase the included splitter...
{F6442591}

...and how the combobox looks
{F6442596}

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