Summary: Use a kcfg file to specify virtual desktops settings saved to kwinrc.
Test Plan:
kcmshell5 kcm_kwin_virtualdesktops
Change something, check apply buttons works appropriately
Save the settings and relaunch the kcm, the new settings are taken loaded correctly
Reviewers: #kwin, ervin
Reviewed By: ervin
Subscribers: kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D25871
