GTK: Properly ask wText what size it wants to please GTK 3.20
It's not really of any use as we do know any size would do as wText is
ours anyway, but GTK 3.20 doesn't like allocating without querying the
preferred size beforehand, so do it.
As wText has a size_request() of 100x100, this might change how we
allocate in case we used to underallocate it, but AFAIK we don't, and
it is the real minimum size expected.
X-Scintilla-Bug-URL: https://sourceforge.net/p/scintilla/bugs/1825/
X-Scintilla-Commit-ID:
d06e3db3e26842cd136328df17eb6f864b3adc02