Ensure the layoutChanged DBus signal gets emitted when changing layouts through DBus

Summary:
Unfortunately Xkb does not emit a signal when the keyboard layout
changes. Due to that we need to manually check in KeyboardLayout after
each action which could change the layout whether the layout changed.

This was not yet done for the case when the layout got changed through
the DBus interface. Resulting in the DBus signal not emitted.

This change addresses the issue by invoking the check for change after
changing the keyboard layout.

Test Plan: Added test case

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D4387