Fix extraction of translatable strings with context using Q_

Q_('ctxt|msg') expands to g_dpgettext(NULL, 'ctxt|msg',0) and should be
interpreted as a message with message context. According to
https://www.gnu.org/software/gettext/manual/gettext.html#Language-specific-options
this glib extension is supported using the 'g' suffix to the keyword spec.
This was missing in our xgettext invocation.

po/gnucash-pot.cmake

@@ -22,7 +22,7 @@ execute_process(
                         --flag=g_scanner_warn:2:c-format
                         --keyword=_
                         --keyword=N_
-                        --keyword=Q_
+                        --keyword=Q_:1g
                         --keyword=translate:1,1t
                         --keyword=translate:1c,2,2t
                         --keyword=translate:1,2,3t