startplasmacompositor: If available, query org.freedesktop.locale1 for XKB defaults

Summary:
kwin_wayland will then use those values to set the default keyboard layout
accordingly.

BUG: 388249

Test Plan:
Ran startplasmacompositor, keyboard layout now set correctly and env
shows all XKB_DEFAULT_* variables set.

Reviewers: #plasma, graesslin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9512