Add Scene button and tab scrolling

[kitbdev]

Fixes #12900
Adds a Plus icon to the right of the last scene tab.
When the tab bar is full, the icon moves to be stationary right before the distraction free mode button.
Click on the icon to add a new scene.
Also, mouse wheel scrolling in Tabs.

[djrm]

i would dim the plus icon a bit

[groud]

And maybe with a little bit darker color :)
Good job anyway ! 👍

[kitbdev]

Updated to dim the icon. Uses Color(0.6f, 0.6f, 0.6f, 0.8f)

[reduz]

@djrm feel free to merge if you are happy with it

[djrm]

some issues:
when you have show scripts enabled this happens, but only when there is one tab

also on certain editor sizes configurations this happens

notice where the plus icon is.

also i could not get the scroll to switch tab functionality to work (is there a setting)

[kitbdev]

Fixed both issues.
The mouse wheel scrolling helps navigate when the tab offset buttons are visible; when you can't see all of the tabs. It doesn't actually select another tab. (it's like scrolling to look at other tabs in FireFox)

[djrm]

@ianb96 ok great.

[djrm]

@reduz also i cant merge, so someone else has to do it

[vnen]

The + changes position if I restore/maximize the window and there's not enough space in the small window to show all tabs.

[kitbdev]

It turns out maximizing the window doesn't emit the resized signal like I thought it would. I update the scene tabs on NOTIFICATION_RESIZED now and the issue is fixed.

[vnen]

Oh, I think it should emit the resized signal when maximizing the window. But I guess this is out of the scope of this PR.