fix(vim/#2968): Cursor management in command-line mode

[bryphe]

Issue: There were a couple related issues with the cursor in command-line mode:

1. When exiting command-line mode, the cursor could 'flash' at the top of the file
2. When running a search command, the viewport would not scroll to the first match

Defect: Both issues stem from the fact that Onivim was ignoring editor-cursor-position changes during search.

For the first issue, when exiting command-line mode, there would be a brief transitional state where the editor is focused in CommandLine mode, but no cursor would be available - so it would be rendered at the default (0,0) position. This would be rectified once the mode transition completed and the cursor position is reset.

For the second issue, libvim was properly updating the cursor position when searching, but that cursor position was being ignored.

Fix: Track the editor cursor position during the course of the command-line interaction

Fixes #2968
Related #1551