This patch adds set of functions to enable quick editing of strips.

• New property Selection Influence for tool Blade with options:
  • Split selected strips
  • Split one strip
  • Split all strips
• New operator sequencer.split_box and tool Scissors - draw 2D box, slpit all strips within and delete them. Operator supports snapping to edit points
• New gizmo SEQUENCER_GGT_preselect_image_preview which will make playhead follow mouse cursor, so user can actually see where edit is being done.

Both tools use gizmo to make playhead follow mouse cursor. Perhaps I could have added option to disable following mouse cursor.

Here is a quick video:

Hacks:

• Gizmo implementation is a bit hacky - it stops working when modal operator starts running, so function is duplicated in sequencer_split_box_modal. I suspect, that I implemented this "other way around".
• Ideally we wouldn't need to move playhead, but either draw preview in timeline directly, but that can be distracting and messy, or in preview area by other means, but that may involve hacks as well.
• Not sure if I should add gesture keymaps like this, or there is more discrete way to do it - see Gesture Box Snap
• Snapping implementation feels a bit hacky as well - see fake_event used to override values for WM_gesture_box_modal

This patch could be split into 3 parts, but it was easier for me to work on it as a whole.