Port from KComboBox to QComboBox
Summary: None of Dolphin's current usages of KComboBox use any of its additional features beyond QComboBox, so let's use QComboBox instead.
Test Plan: Tested all features that use comboboxes in Dolphin. No visual or functional changes or regressions.
Reviewers: Dolphin, elvisangelaccio, broulik
Reviewed By: Dolphin, elvisangelaccio
Subscribers: broulik, kfm-devel
Differential Revision: https://phabricator.kde.org/D13749