Merge branch 'fullscreen-buttons' of https://github.com/ochosi/xfce4-screensaver into ochosi-fullscreen-buttons

Somehow this isn't enough.

Improve panel

The .panel and .xfce4-panel style classes will both be picked up to
style the panel more nicely by default.
Also, the quite hectic time in seconds is changed to a time in minutes,
which should provide enough detail for a simple (un)lock screen.

Drop leave-message feature

Also cleanup unneeded widgetry (GtkNotebooks etc).

Improve layout and theming

These theming tweaks should work with all themes that support
lightdm-gtk-greeter (like Greybird).