Restore shortcut schemes editor

Summary:

• Port shortcut scheme file search to KoResourcePaths Also prettify the shortcut scheme options

• Update comments+indentation in KisShortcutsEditor

• Restore changeShortcutScheme()

• Move shortcut schemes work from XMLGUI to KisActionRegistry.

• Define behavior of custom shortcuts vs. shortcut schemes kritashortcutsrc now contains only custom shortcuts. Shortcut schemes files are loaded separately and go in share/krita/shortcuts/

• Add empty (for now) directory for shortcut schemes This directory is where the shortcut editor will look for scheme files.

Reviewers: Krita:_next

Differential Revision: https://phabricator.kde.org/D648