gtkcombobox: in has-entry case, allow entry be shorter that 150px

When "has-entry" is true (GtkCombobox has an editable GtkEntry)
allow the GtkEntry to be shorter than 150px and thus match the
ComboBox width when this is shorter than 150px.

This fixes evince#1002 by work around the issue #1422 for the
GtkComboBox case.
Status Job ID Name Coverage
  Build
passed #129572
build

00:08:19

manual #129573
win32 allowed to fail manual
msys2-mingw32