libgimp: allow NULL title for gimp_procedure_dialog_new()…

… but only when a menu label was set with
gimp_procedure_set_menu_label(). In such case, this menu label is used
as dialog title (with mnemonic underscore removed).
16 jobs for master in 44 minutes and 18 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Prepare
passed #1123184
build-image

00:01:54

passed #1123186
build-image-win32

00:00:33

passed #1123185
build-image-win64

00:01:14

 
  Dependencies
passed #1123187
deps-debian

00:01:39

passed #1123189
deps-win32

00:16:09

passed #1123188
deps-win64

00:16:43

 
  Gimp
passed #1123190
gimp-autotools-debian

00:08:42

passed #1123193
gimp-clang-debian

00:07:20

passed #1123191
gimp-distcheck-debian

00:22:45

passed #1123192
gimp-meson-debian

00:08:39

passed #1123195
gimp-win32

00:14:01

passed #1123194
gimp-win64

00:10:05

 
  Analysis
passed #1123196
cppcheck

00:02:18

 
  Distribution
passed #1123197
sources

00:01:05

passed #1123199
win32-nightly

00:13:30

passed #1123198
win64-nightly

00:09:47