Blender Git Loki

VSE: Remove separator lines between rows

The VSE grid theme setting is currently used for two things:

* Indicate time intervals (vertical lines)
* As separator between channels (horizontal lines)

This adds visual noise because for the time interval to be visible, the
grid color needs to be bright, resulting in a rectangle-grid backdrop.

Recently, the VSE got a theme setting to customize alternate-row background color.
This should be sufficient to tell the channels apart without the need for a line in between.

Additionally, this patch makes the VSE background use the theme setting as-is,
without hard-coded darkening, to ease the tweaking of themes. This aligns the style
of the VSE backdrop with the rest of Blender (Outliner rows, File Browser, Spreadsheet,
Info and animation editors).

Related reports: T92581
Related task: T92792

#### Before
{F11680317, size=full}

#### After
{F11694981, size=full}

Reviewed By: #user_interface, Severin

Maniphest Tasks: T92581

Differential Revision: https://developer.blender.org/D13072