Fix alarm service PV handling for PVs with leading slashes

[DKastelic]

Fixes #3612 by disallowing PVs (and nodes) with multiple leading slashes.

Trying to add a PV/node with multiple leading slashes raises an exception, which is handled by the GUI by opening an error dialog
and by the XmlModelReader by logging a warning.

Also fixes a bug where moving an alarm tree node to the same path it already has would delete it.

Checklist

• Testing:

  • The feature has automated tests
    • Copied the AlarmTreePath unit test from the old CS Studio to Phoebus and added a test case for the new exception.
  • Tests were run
    • Made sure the cases described in Alarm Tree breaks on duplicate PVs with slashes #3612 are now handled appropriately.

• Documentation:

  • The feature is documented
    • The new exception has a javadoc comment and makePath has an added @throws tag
  • The documentation is up to date
  • Release notes:
    • Added an entry if the change is breaking or significant
    • Added an entry when adding a new feature