Split bugfixes and user interation.

- bugfixes for splits
- added some asserts
- showDebug is now taken from the script's #debug label
- finalized user interaction (initializing search from selected text or
  previous search...)
- changed all key events to keydown
- fixed word highlight overriding search highlight in codemirror

The only missing part of the main functionality is starting the search
from the current cursor position.