xfce4-xkb-plugin: Add package

Allows you to setup and use multiple keyboard layouts.
xfce4-datetime-plugin: Add package

Shows the date and time in the panel, and a calendar appears when you left-click on it.
xfce4-session: Don't set $PATH in xflock4 script

This makes Xfce4's "Lock Screen" action button actually work when xscreensaver
is installed and the daemon is running.
Merge pull request #1205 from wizeman/xfce

Xfce improvements