Citation dialog: Fix trouble with openbox, fluxbox

Citation dialog: Fix trouble with openbox, fluxbox

http://thread.gmane.org/gmane.editors.lyx.devel/161725

These wms have trouble with the fix at b5a2f1c7, probably more precisely with
the trick to force the calculation of the actual sizes before the display
(layout->invalidate() and the code around it).

This patch gets rid of the code that forces the calculation. As a consequence,
the minimum sizes are again incorrect the first time the window is shown. They
are only correct the second time the window is shown. Now here is the trick: LyX
remembers the sizes of windows between sessions. Therefore, as soon as the good
minimum size has been set, the good size is remembered for the next
session. Thus, in the following sessions, even though the minimum size is
incorrect the first time, the dialog still opens with the good size. So the user
does not see the problem in practice, apart from the very first time.

This is meant as a temporary workaround.