Fix keyboard shortcuts in automatically generated actions (fixes #408).

Having our own "copy" menu did cause the automatically generated action
of the QTextEditor used as console to drop the CTRL+C link. Using the
WidgetWithChildrenShortcut settings seems to fix that for Qt4 and Qt5.
Merge pull request #770 from openscad/copy-paste-fix

Fix keyboard shortcuts in automatically generated actions (fixes #408).