Make editor dock tabs take up the whole width

[alexfreyre]

Describe the project you are working on

Some Godot UI details and the shortest PR ever!

Describe the problem or limitation you are having in your project

Why do we leave space?

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Let's make the tabs take up all the space around! They deserve it!

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

If this enhancement will not be used often, can it be worked around with a few lines of script?

According to @Calinou "this functionality must be implemented in TabContainer"

Is there a reason why this should be core and not an add-on in the asset library?

I think it is core!

[Calinou]

Why do we leave space?

This is an interesting question, but you'll notice that in popular GUI apps, leaving out space around tabs is pretty common. For instance, none of the major web browsers will use up all the horizontal space available when you only have a few tabs open.
(I think Safari on desktop stretched tabs to full width for a long time, but I remember them switching to a more traditional layout recently.)

Wider tabs are easier to click, but due to their varying width, they suffer from many usability issues:

• The tab width is "unpredictable" for a casual user and may slip around when you're trying to close many tabs in a quick sucession.
• The tab's text position will also be unpredictable in the same way, which means the user will have to spend more time to "locate" the text. This isn't really an issue in narrow docks, but it can be an issue for wide containers like the main viewport (which uses Tabs, not TabContainer).

[alexfreyre]

This proposal don't apply for scene tabs above the viewport!

• The other tabs can divide the space:
  1 = 100%
  2 = 50%
  3 = 33.3%

[vntxx]

How about a separate tab alignment option? I think something like "Fill" would fit quite nicely. It could then be applied to specific docks in the editor.