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