[Logout screen] Change labels to match SDDM theme labels
Summary:
This patch changes the logout screen labels to match the labels in the SDDM theme.
Reboot --> Restart
Logout --> Log Out (Title Capitalization)
It also adjusts the corresponding messages that use the verbs.
Reviewers: VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D19458