Add tab groups

Tabs can be assigned now to a group by right-clicking on it. Uses:

- Tabs on groups share the timeline only with other tabs on the same group,
  regardless of the 'shared timeline' settings.
- Added a new 'enable scroll wheel through tabs on the same group' setting,
  similar to the already existing 'enable scroll wheel through similar tabs'.
  Both settings are moved from the 'misc' to the 'video' tab on the program
  settings.

Warning: session files produced after this commit won't be compatible with
past releases. Changed also how this info is stored to prevent it from
happening again.