GTK: Fix auto-completion popup row height computation on GTK 3.14 GTK 3.14 changed how the cell padding is calculated, and I can't seem to understand the new logic yet.  So, use the correct API for computing the row size, which unfortunately doesn't work for us on GTK2, but otherwise is a better way to go anyway.
GTK: Fix auto-completion popup row height computation on GTK 3.14
GTK 3.14 changed how the cell padding is calculated, and I can't seem
to understand the new logic yet.  So, use the correct API for computing
the row size, which unfortunately doesn't work for us on GTK2, but
otherwise is a better way to go anyway.

Tested with GTK 3.2, 3.8, 3.10, 3.12 and 3.14.

windoze/scintilla,windoze/scintilla,windoze/scintilla,windoze/scintilla,windoze/scintilla,windoze/scintilla,windoze/scintilla,windoze/scintilla