Improve "Add" button consistency in Project Settings

[ckaiser]

There's a few inconsistencies I've noticed while using the project settings window, mainly in places where we have a textbox with a button next to them that we're using to "Add" things.

This PR attempts to make all of these tabs behave more consistently and tidy up some minor UX things:

• Adds a plus icon icon to all the "Add" buttons next to text inputs, with the hopes of making them more distinguishable
• Adds a separator next to the action map "add" button for clarity
• Modifies the Shader Globals editor to be consistent with the others (moving the name based error to the button popup, I kept the other errors in the message boxes since I'm unsure if those are more expensive checks that might not be wise to make every time there's a change in the input and they're a bit less obvious than an empty input or a bad character)
• Adds the "Clear" button to the shader globals input since we may not want to clear it entirely when adding a new variable in case we're doing something sequential "var1, var2, var3", but if we're adding multiple variables at time, just clicking on the clear button and typing a new name to add a second variable is convenient.

Before and after screenshot:

I took the liberty of going directly to my first PR without creating a proposal for this because I figured it was a small thing, but I'm not super familiar with how the process goes for things like this usually, since this isn't really a "fix" or a "feature" per-se, and I have some more ideas for things I'd like to implement on the UX side of the editor, mostly minor like this, would making a proposal for several changes for a single window/panel/widget make sense or should I just keep making direct PRs as long as the changes are minor and in line with how things in the editor usually work?

[KoBeWi]

Global Groups tab also needs similar treatment (it was added after this PR was opened).

[ckaiser]

Global Groups tab also needs similar treatment (it was added after this PR was opened).

I can tackle that in another PR then :)

[KoBeWi]

It should be in this PR, it's part of the same dialog.

[ckaiser]

It should be in this PR, it's part of the same dialog.

Alright, done! Updated the branch as well

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉