Summary:
Saving Mirror Tool state to KRA file, wished for in bug 339515.

BUG:339515

Test Plan:
  - Set up mirror lines. Save, close and reopen. Expected result: the mirror lines and options should be automatically set and activated.
  - Open multiple files, with and without mirror lines. Expected result: Every open document should have separate configuration. Toolbar actions should affect only the currently active view.

Reviewers: #krita, rempt

Reviewed By: #krita, rempt

Subscribers: rempt

Tags: #krita

Differential Revision: https://phabricator.kde.org/D18254