Added an EditorState class

editorstate.py - new file

editorperspective.py - modified

timeline/timeline.py - modified

utils/timeline.py - modified

test_editorstate.py - new file

common.py - modified

Users want to resume editing with the same interface state.

The playhead position, zoom level, scroll, and selected clips are lost.

We added editorstate.py which is used to save and restore these values.

Closes GitLab issue #2029 (closed)