Deprecate the ToolButton node

[YeldhamDev]

ToolButton is a Button that has flat enabled by default. That's it.

I don't think such a thing needs a node of its own, and would probably be for the better if it was marked as deprecated and then removed in 4.0.

But wait! It's heavily used in the editor itself!
Indeed, and it saves a total of one single line. Sure, the editor has lots of buttons, so those single lines pile up, but here's a better solution, making Button take a argument when created, like Label does:
Label *new_label = memnew(Label("Placing text here avoids needing a extra line."));

[KoBeWi]

I love how single copyright header is longer than whole declaration and implementation of this class XD

Constructor argument makes sense, but is flat really that important option to be there? (I mean outside editor ofc)
EDIT: Ok, maybe this doesn't matter with the overloading.

[groud]

Making ToolButton an editor-only feature makes even more sense IMHO.

[akien-mga]

Should we simply make the editor theme have flat by default for all buttons, and drop ToolButton + the manual set_flat()s? Or are there good use cases for both flat and non-flat buttons in the editor?

[groud]

Should we simply make the editor theme have flat by default for all buttons

That would make sense, but for now the "flat" button property is not overridable in the theme. It should be made an overridable constant.

[YeldhamDev]

There are a lot of non-flat buttons (mainly the "Cancel" and "OK" in dialogs) that would look odd if flat, so I don't think setting all buttons to one option would work.

[groud]

Indeed, if we move that to a theme constant, it can also be overriden later on for specific nodes.

[Calinou]

For the record, the editor theme redesign I'm currently working on doesn't need the flat property, so I already removed it there. ToolButton is still present in that branch, but it's identical to a Button otherwise (because I didn't bother replacing it yet).

[YeldhamDev]

Closed by #39690.