Set window size to default when unfullscreening.

This works around problems in xfwm4 (maybe others?) where tabbing out of tilda 
(e.g. to change desktop) when fullscreened will mean a subsequent unfullscreen 
won't restore the dimensions of the window.

I'm sure there must be a better way to do this but I can't see any obvious point 
where the dimension info is lost so I used the brute force method.  I'm guessing 
that this patch will have the limitation that if you set a custom dimension then
it won't be restored properly.

So consider this my best guess which solves my immediate problem.

Here are the steps to reproduce the error:

* ctrl+f1 = go to desktop 1
* f1 = drop down tilda
* f11 = toggle fullscreen
* ctrl+f2 = go to desktop 2
* f1 = minimise tilda [1]
* f1 = drop down tilda
* f11 = unfullscreen

Outcome: position has been restored, but not dimensions.

If you do not minimise tilda [1] then unfullscreen works as expected.  If you change the width in the settings then it will fix the problem too.

Anyone got a better fix?