Remove Doxygen.

Fixes #5006
Documentation: Update cluster documentation.

Fixes #6234
Refs #6192
Refs #6191
Use CustomVarObject instead of DynamicObject for all libicinga objects.

Fixes #6187
Add MessageOrigin to OnVarsChanged.

Refs #6187
Refs #5219
Documentation: Add a note for required CheckCommand objects for additiona plugins.

Fixes #6239
Move 'GraphiteWriter already connected' to debug log.

Refs #6070
Test.