cursor position in split editor panes

[mvolkmann]

Is this a BUG REPORT or FEATURE REQUEST? (choose one):
BUG REPORT

What happened:
Cursor position is not maintained separately for each split pane.

What did you expect to happen:
Each pane should have its own notion of the current cursor position.

How to reproduce it (as minimally and precisely as possible):

1. Open any file.
2. Click button in upper-right to "Split Editor".
3. Move cursor down a few lines.
4. Click in other split pane.
5. Move cursor down one line.
6. Notice that it moves down one line from the position in the previous pane,
   not from the position in this pane.

Environment:

• Extension (VsCodeVim) version: 0.12.0
• VSCode version: 1.23.1
• OS version: Windows_NT x64 6.1.7601

[jpoon]

Dupe of #2688