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

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D13749