Allow prefixing with '%' to set a node as unique in the Scene Tree Dock

[Synzorasize]

Closes godotengine/godot-proposals#11386. This allows users to set any node (except for root nodes) as unique while renaming it by simply prefixing with "%". Feel free to request changes.

[yankscally]

so cool! wowee! love me some QoL

[badsectoracula]

I tried this and it works as described. The code looks ok too.

There are two things that i noticed:

1. Trying to rename something to '%Foo' when there is another unique 'Foo' elsewhere gives some internal warning. Perhaps it could give an error message or produce a new name like if you try to rename something to 'Foo' when there is already an existing 'Foo' (in which case the node is renamed to 'Foo2'). Note that this already happens if you try to rename something to '%Foo' with an existing 'Foo' under the same parent.
2. I expected that if you try to rename an already unique node, the editable field would add the '%' so you can either remove it, thus making the node non-unique. Technically the PR is about making a node unique, not the reverse, it just feels a bit inconsistent :-P. Also it'd help with just pressing F2 and then Ctrl+C to copy the unique name for use in scripts.

[KoBeWi]

Trying to rename something to '%Foo' when there is another unique 'Foo' elsewhere gives some internal warning.

The worse part is that this discards your name. Showing warning is probably fine, but the node should still be renamed, just without unique name.

[Synzorasize]

Thanks for the suggestions, @badsectoracula and @KoBeWi!

1. I was going with the original implementation for if there was already a unique name, but I do agree it makes more sense to keep the name if not make it a unique name. I also think renaming the node to "Foo2" makes sense, to ease the workflow further. Since the user would now be able to set a unique name just by renaming the node, it would be the most intuitive and consistent solution here as well. But perhaps someone else can give their opinion about this?
2. This is pretty similar to @Mickeon's suggestions in Improve how Scene Unique Nodes are highlighted in Scene Tree Editor godot-proposals#4494, which I took a look at. I'm not sure if that would be out of scope for this PR, since it just adds a new feature, not changing any visual appearance. But it can always be implemented in a new PR if we want. For the keyboard shortcut, I was thinking while renaming it, the user could Ctrl/Cmd+A Ctrl/Cmd+C (selecting the entire field and copying) it before confirming so that it can be pasted into code. But I suppose if you want to copy with the "%" even after renaming it, this makes sense.

[Synzorasize]

It looks like renaming to "Foo2" would either need some changes in scene/main/node.cpp since it currently only supports renumbering of child nodes, or duplicating the similar logic to the function, which would make it very complex. So that is probably out-of-scope for this PR. But perhaps it can be implemented in a future PR. For now, I will use @KoBeWi's suggestion.

Also fixed a bug where adding "%" without changing the rest of the name while there is already a unique node with the same name does not show a warning.

Edit: Also rebased from master. That's why the comparison between force-pushes way more complex than it should.

[KoBeWi]

Changing node name to (different) unique one will create 2 undo actions. I think it should be one.

[Synzorasize]

Changing node name to (different) unique one will create 2 undo actions. I think it should be one.

This makes sense since only one action was done by the user. I decided to set them as one action.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉