VSE: Add drag and drop handler for preview area

For some users, dropping assets into preview area may be more practical
due to space constraints or it may be just more intuitive.

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D13311

VSE: Support drag and drop for datablocks

For using the Outliner and/or the Asset Browser as scene independent
tools to organize a/v source material is necessary for the users to be
able to drag and drop data blocks into the VSE. This was also an
unfulfilled design target for the Outliner Gsoc project.

Datablocks won't be used directly. Path to file will be passed to strip
add operator instead.

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D13304

VSE: Fix of several outline and handle related problems by bracket handles

There are numerous problems with the current handles and outlines:
- Current handles are making waveform editing imprecise: T90824
- They are not showing a direction(previously they had arrows).
- Having both strip(via outline) and handles selection drawn is the cause of confusion: T90843
- Strip overlay texts are stealing attention away from with the selected elements.
- Adjoined outlines are drawn op top of each other.

Solutions:
- Change handles to bracket shaped(introduced in: D10297), so the waveform is not covered by the handle.
- Selection is drawn as either handle or strip, so there is no confusion(and it is in consistency with ex. 3d view where a selected object and a selected vertex are drawn at the same time).
- Since the handles are smaller this way, the overlay and transform texts colors are muted a bit, so selection will stand out as the most highlighted elements.
- The outlines have been tweaked, so the outlines are moved away from each other and a dark line is separating them.

Before
{F10768098,size=full}

After
{F10768103,size=full}

A gif:
{F10768170,size=full}

Differential Revision: https://developer.blender.org/D12308

VSE: Clamp resulting frame in multiply mode

The clamp added will ensure immediate speed direction change on
changing to/from positive/negative speed factor when using the Speed
effect strip's Multiply mode.

Reviewed By: ISS, sergey

Differential Revision: https://developer.blender.org/D12462

VSE: Expose Zoom to Fit in all display modes

Zoom to Fit is working in all display modes, but was only exposed in
Image mode. This patch exposes it in all Image modes.

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D12632

VSE: Add bold and italic option for text strip

Bold and italic fonts can be switched quickly by presing corresponding
button.

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D10542

VSE: Text strip improvements

- Position text in center of image
- Increase default font size so it's more visible
- Increase font size limit
- Increase limit for location, so text can be scolled off screen
- Wrap text by default
- Tweak default box and shadow color
- Change text shadow position
- Text box no longer casts shadow

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D10571

VSE: Preview images when moving strip handles

Add option to override current frame whem transforming strip handles.
Option can be found in View menu of VSE preview, or in timeline when
using backdrop.

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D10424

VSE UI: Cleanup scene panel style

- Make properties alligned
- Add decorate to volume
- Add heading to Grease Pencil and Transparent

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D10663

VSE: Fix duplicate menu entries

Commit rBf448ff2afe7a accidentally added menu entries that were removed.
`use_zoom_to_fit` is duplicated in menu, others were moved to overlays menu.

Remove these entries.

Reviewed By: ISS, campbellbarton

Differential Revision: https://developer.blender.org/D10453