GUI: call G_gisinit when forms are run standalone (e.g. from parser) (O…

…SGeo#2489)

* GUI: call G_gisinit when forms are run standalone (e.g. from parser)
Repeated calls to G_gisinit are safe;
When forms.py are loaded as a part of other code, one can assume that
G_gisinit has already been called.

See OSGeo#2484 for discussion.

gui/wxpython/gui_core/gselect.py

@@ -3109,6 +3109,14 @@ def __init__(
         self.SetValue("")
 
     def _append_mapset_signatures(self, mapset, element, items):
+        # A workaround to list signature files before a separate
+        # signature management module is developed
+        try:
+            from grass.lib.gis import G_gisinit
+
+            G_gisinit("")
+        except Exception:
+            return
         try:
             from grass.lib.imagery import (
                 I_SIGFILE_TYPE_SIG,