fix config dialogs in ltr mode

Summary:
anchor the layouts of config dialogs to left
this way they will look the same normally (they
are positioned at x 0 normally) but will be
anchored to right instead if layout mirroring is enabled

CCBUG:372721

Test Plan: opened all config dialogs both in ltr and ltr layouts

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3456