Merge branch 'rot-invariant'

This implements the rotation invariant duplicates search, implemented by Cyril
Roussillon.

The Feature is enabled by default but can be disabled in preferences dialog.

Please note that, due a slightly different fingerprint algorithm, the .sim files
needs to be deleted manually and recreated to get the full benefit.

* rot-invariant:
  Allow to configure rotation invariance
  Slightly better similarity samples
  Implementing rotation invariant duplicates search