WIP add key shortcut to switch between editor for current row

This adds a key shortcut ctrl+alt+h to switch between codemirror and plain text for current row. It also changes default editor to plain text. 

- It will help people relying on browser spell checker to work as well as having syntax highlight available at fingertips. https://bugzilla.redhat.com/show_bug.cgi?id=1062508
- We may consider changing source panel to use codemirror runmode (like what's in TM) so users can right click with copy & paste option available. https://bugzilla.redhat.com/show_bug.cgi?id=1024130 

I mark this PR as WIP because of point 2 and we may want to remove code mirror editor entirely (just use runmode + plain text). May clean up the styling a bit at the same time. Open for discussion and comment.