Selections are not stored in the text operations for Undo

[MattUV]

Godot version:
2.1.4, 3.0 current master a714e1a

OS/device including version:
Windows 8.1

Issue description:

• In the script editor, or a the TextEdit node, when you undo text editing, the cursor moves where it was at that time, but not the selection.

• If you undo while text is selected somewhere else, the selection remains.

• If the selected text moves when undoing, the selection will keep its global position, and thus, will not keep the same words selected.
  (Fixing that will fix the last issues in Multiple problems with TextEdit selection while indenting #14559)
  

[zqyoung1]

Is this topic still open? If it is I'd like to work on it.

I'm a novice contributer to godot so if this is a problem please let me know!