Fix generator effect strips not overwriting

Add overlap handling mode to header

[WIP] VSE: Transform overwrite mode

WIP status: logic is implemented, and overwrite mode is currently forced.

Add mode to overwrite strips on overlap instead of resolving overlap.
When overlap is created, 3 things can happen:
- On partial overlap, handles of overlapped strip are moved
- On complete overlap with smaller strip, overlapped strip is split
- On complete overlap with larger strip, overlapped strip is removed

There are some edge cases where this mode may not work very well, this
is mainly case for partial overlap with effects. This situation can be
resolved in more than one way and I don't think it can be predicted.

Differential Revision: https://developer.blender.org/D11805

Fix T89722: Duplicate macro can cause strips to overlap

Bug caused by 78693d524c13 accidentally removing overlap handling when
transform operator is canceled.

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D11899

VSE: Fix multicam splitting all selected strips

`split_multicam` used split operator, where if more strips than
multicam were selected, all would be split, which is undesirable.

Add `Sequence.split()` RNA API function. to split individual strips.
Function accepts `frame` and `split_method arguments`. Returns right
strip after splitting.

In case when strip being split have effects, these will be split too, so
no invalid state should be created.

Selection is not handled, this is by design up to user.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D11926

Display only major ticks, update theme, so appearence is identical to previous version

VSE: Don't draw time grid lines

Add overlay option to disable grid drawing.
Reuse drawing code from other editors (timeline editor )

{F10215219,size=full}

Differential Revision: https://developer.blender.org/D11790

Fix T88908: Incorrect path handling in adding strips

When image strip is added from python using `image_strip_add` operator
and directory path is not terminated with slash, last part of directory
was ignored.

Use `BLI_join_dirfile` instead of simple string concatenation.

Fix T89826: VSE snapping with constrained axis

Constraining to X axis caused snapping to not work at all. Constraining
to Y axis caused snapping indicator to be drawn, when snapping doesn't
occur.

Reviewed By: mano-wii

Differential Revision: https://developer.blender.org/D11898

VSE: Make pasted strip active

When adding texts or various simple effects I often copy-paste strips
to reuse properties from a template such as font or position. I assume
this is common workflow. Issue with this workflow is, that active strip
is not changed after pasting, so when adjusting property, it is original
strip that is being modified.

This is not issue when duplicating strips - selection state is
transfered to duplicate strips, such that duplicate of active strip is
set to be active and duplicate of selected strip is set to selected.

Implement same selection transfering behavior in paste operator, that
exists in duplicate operator.

Since strip can be deleted after copying, it is not possible to rely
on sequencer state. This is true even when pasting strips to different
scene. Therefore active strip name must be stored in clipboard.

Reviewed By: sergey, Severin

Differential Revision: https://developer.blender.org/D11781