Make the focus outline translucent for editor viewports

[Calinou]

This makes the focus outline less distracting on the 2D and 3D editor viewports. We still need to keep this outline visible as it's the only way the user can see if their key presses will go to the viewport (for actions like Focus Selection). Originally requested on Twitter.

Note: Not cherry-pickable to the 3.x branch as this only pertains to the new editor theme.

Preview

Note: These screenshots are taken at the smallest possible window size. Remember that the focus outline becomes less distracting at larger screen sizes, since it takes up a lower portion of the window.

Before

After

[samdze]

I prefer the non-translucent version for color consistency reasons, but not a big deal.
The "distracting" issue here is magnified due to the very small editor window in the preview.

[sairam4123]

Make sure it's possible to change Translucency in the editor settings.

[Calinou]

Make sure it's possible to change Translucency in the editor settings.

I wouldn't add a setting for this as it's too niche. Instead, we should focus on picking a good default value, and the new default seems to be good enough.

[sairam4123]

Make sure it's possible to change Translucency in the editor settings.

I wouldn't add a setting for this as it's too niche. Instead, we should focus on picking a good default value, and the new default seems to be good enough.

Well, I liked the old opaque one, we could possibly range the setting between 0.25 and 1.

[aaronfranke]

Tested in combination with #35891 and it looks great to me! This should be merged before #35891.

I'm tempted to suggest that this should just apply to all instances of the focus outline, but I'm not sure.

[akien-mga]

Needs a rebase.

[Calinou]

Rebased and tested again, it works as expected.

[akien-mga]

Thanks!