Move search and filter bar close buttons to the right

Summary:
Close buttons for windows, tabs, and pretty much everything else in KDE are on the right,
not the left. This patch makes Dolphin's search and filter bars follow that.

BUG: 421372
FIXED-IN: 20.08.0

Test Plan: {F8314176}

Reviewers: abetts, #dolphin, #vdg, elvisangelaccio, GB_2

Reviewed By: #dolphin, #vdg, elvisangelaccio, GB_2

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D29693