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