For issue 395, implemented most of a undo/redo mechanism
Bug fixes in subtitle undo/redo mechanism
Bug fixes and additional tests for undo/redo mechanism in SubtitleList
For issue 395, also trigger callbacks when undoing/redoing, bug fixes, and more tests
Fixes case in method names for undo/redo in editor
Merge pull request #1145 from syl22-00/gh-395

Include undo/redo feature Gh 395