tdf#78111 Sane preferred width for the color mode combobox

Don't let long strings (in some languages) make it request so much space.

Change-Id: I5b2af9d7cc54d1ba772ae75971cfff27bee31a05
(cherry picked from commit d2f218f5)
Reviewed-on: https://gerrit.libreoffice.org/17699Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svx/source/sidebar/graphic/GraphicPropertyPanel.cxx

@@ -58,6 +58,7 @@ GraphicPropertyPanel::GraphicPropertyPanel(
     get(mpMtrBrightness, "setbrightness");
     get(mpMtrContrast, "setcontrast");
     get(mpLBColorMode, "setcolormode");
+    mpLBColorMode->set_width_request(mpLBColorMode->get_preferred_size().Width());
     get(mpMtrTrans, "settransparency");
     get(mpMtrRed, "setred");
     get(mpMtrGreen, "setgreen");