Fix typos

Summary: This diff only contains a few typos I noticed over time, and the removal of misspelled "AbstractItemVew" forward declarations, which don't seem to be needed since they aren't written correctly anyway.

Test Plan: Nothing, just needs to build

Reviewers: O1 MITK Reviewer Group I, O3 MITK Reviewer Group II, floca

Reviewed By: O1 MITK Reviewer Group I, O3 MITK Reviewer Group II, floca

Differential Revision: https://phabricator.mitk.org/D713