Create constrain angle action
This adds a KToggleAction which sets annotation tools to constrain angle mode. It provides an alternative user interface to the Shift button, which is used to constrain angles since MR !210 (merged). The action and the Shift button are XOR-ed, i. e. if constrain angle mode is activated, pressing Shift temporarily disables it.
The action state is remembered accross sessions, for consistency with most other actions. It should be difficult to check this action without knowing of its existence, since it is not in any toolbar or menu, just in the action collection.
The meaning of AnnotatorEngine::Modifiers was generalized a bit, moving the responsibility about whether to constrain angles back to PageViewAnnotator, because AnnotatorEngine does not know about the action.