Allow to set tooltip in command applet

[develheini]

This patch allows the script for the Command applet to display additional information when the cursor is moved over the applet. This is done by setting the Key "Tooltip" in the scripts output. Example output:

[Command]
Output=Ok
Tooltip=All checks successful!

[vkareh]

This works well on my side.

Easiest way of testing is by making this the command (with newlines and all):

echo "[Command]
Output=foo
Icon=mate
Tooltip=All checks successful!"
``

[vkareh]

Another thing - I didn't even know that using a GKeyFile as the command output was possible. Any chance you can add a README under the command applet directory, or some sort of help file? Doesn't need to be in this PR, it can be done after too.