SpellCheckBar: Use DictionaryComboBox instead of plain QComboBox

Summary:
This patch alone is reasonable but become really important with the aim of D17030
and complies to D17056

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: mludwig, kwrite-devel, kde-frameworks-devel, KTextEditor

Tags: KTextEditor, Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D17103