ToolButton has no redeeming differences with Button;
it's just a Button with the Flat property enabled by default.
Removing it avoids some confusion when creating GUIs.
Existing ToolButtons will be converted to Buttons, but the Flat
property won't be enabled automatically.
This closes https://github.com/godotengine/godot-proposals/issues/1081.
Which means that reduz' beloved style which we all became used to
will now be changed automatically to remove the first empty line.
This makes us lean closer to 1TBS (the one true brace style) instead
of hybridating it with some Allman-inspired spacing.
There's still the case of braces around single-statement blocks that
needs to be addressed (but clang-format can't help with that, but
clang-tidy may if we agree about it).
Part of #33027.
Remove now unnecessary bindings of signal callbacks in the public API.
There might be some false positives that need rebinding if they were
meant to be public.
No regular expressions were harmed in the making of this commit.
(Nah, just kidding.)
