[xfce4]: clock now a plugin

[Jasper Huijsmans]

Hi all,

I intend to change the clock and trash modules into a plugin. My idea is
to only have builtin modules for icon launchers, desktop switcher (I
still have to make that a normal module), and anything that requires
internal functions (exit, setup).

As a first step I have moved the clock to be a plugin. This means that
the first time you'll start the panel after rebuiding it, if you had a
clock on the panel it will not work now. Just change it by right
clicking on the item and choosing the XFce clock in the dialog.

Jasper

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