Set scope of custom shortcut controller to local

This allows overriding GTKInspector's shortcut (Ctrl+Shift+D) to open the document properties dialog instead. (see #4802, 'Basic' section)

Judging from the documentation for set_scope(), it looks like the Global scope is mainly for global hotkeys, which Inkscape does not use (atleast not as far as I know).

A potential issue is that this requires the Inkscape window to be in focus for shortcuts to work, and I am not sure if floating dialogs cause a problem here. Couldn't really check this as using shortcuts with a floating dialog in focus is already broken in master.