Backport of r39360, r39361 and r39363:

Backport of r39360, r39361 and r39363:
- move spellchecker backend change action to actOnUpdatedPrefs where it belongs to
- add missing action for change of spellchecker_accept_compound preference
- care for a change of accept_compound and reconfigure the aspell backend accordingly
- only aspell is using spellchecker_accept_compound rc variable

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/branches/BRANCH_2_0_X@39471 a592a061-630c-0410-9148-cb99ea01b6c8