Add tab groups, part II

- Add 'apply offsets': if this option is checked the difference between each
  showed frame when the tabs were added to the group is used as an offset for
  sharing the timeline, e.g. tab1 (on frame 100) and tab2(on frame 250) are
  added to a group -> tab2 is always 150 frames ahead when switching between
  the two tabs.
- Add 'clear current tab group' and 'clear all tab groups'
Add tab groups, part III

- Add tab groups submenu to the video menu
- Allow to scroll the video preview if 'enable scroll wheel through tabs on
  the same group' is checked but no 'enable scroll wheel through similar tabs'
  and the current tab is not on a group.
Add tab groups, part IV

- Prefix group number to the tab title
- Revert 10475ed1cdfe as it causes GetModify to be unreliable