[issue #470 - session specific include paths] adding replace checkbox

Give user option to replace or add to global paths.
[issue #470 - session specific include paths] adapt documentinfo to new session settings
[issue #470 - session specific include paths] clean doesn't remove global paths
[issue #470 - session specific include paths] checking replace deletes global paths
[issue #470] rename 'clean' to 'clear'
[issue #470] new functionality and name for previous 'revert' button
Merge pull request #480 from PeterBjuhr/sessions

Sessions - option to add to or replace global include paths
Merge pull request #481 from PeterBjuhr/musicxml

Musicxml export bug fixes
Allow reference of existing context
...without creating a new part