Allow tab_rmb_clicked to always work

[lodetrick]

Currently, the tab_rmb_clicked signal only works when select_with_rmb is enabled. This is not always wanted however, say if you wanted to use the right-click to show a popup. This PR enables the tab_rmb_clicked signal to always be emitted if you right click a tab, even if select_with_rmb is disabled.

Old Comment

Currently, when users add a popup to a TabContainer, it only adds a small icon on the right that they can click to show the popup. Thus, a common use is to allow right clicks to tabs that also brings up the popup. However, the current method to do this is hacky and requires adding another function. The current method is to connect the tabbar's `gui_input` signal to a new method, in a line like: https://github.com/godotengine/godot/blob/d9cd011e2fa9fa9a3011371843729f33032cee35/editor/editor_dock_manager.cpp#L997

This PR adds an optional boolean to the popup method that can enable this behavior natively, reducing the amount of extra code and hacky workarounds. It's disabled by default to preserve compatibility.

I don't know how much the maintainers would want to expose this but I do think that it is a good usability enhancement. Worst case if this doesn't get exposed a boolean could be made with new methods so the existing methods don't get changed.

[kitbdev]

Connecting to the gui_input is very common, I wouldn't say it is hacky at all. I also don't think it is that common to have the popup be the same as the right click menu.
IMO Ideally, the dock select should only be on the right click menu since it depends on the dock, not the dock slot as a whole. But it already was at the TabContainer popup when I added it to the right click and there wasn't anything to replace it with or a good reason to remove it from there.
Changing the mouse filter based on whether this is enabled seems more hacky to me.

If anything we could improve the tab_rmb_clicked signal to not need select_with_rmb and connect to that instead.

Also this looks like it doesn't set the context dock properly, it should work even if the dock right clicked it isn't selected.

[lodetrick]

I'll pivot this PR to focus on that signal then, thanks for the advice

[lodetrick]

@kitbdev I was able to clean up the click checking, and allowed the tab_rmb_clicked to always be emitted when the right button is pressed. I assumed that the close button and right button were always contained inside the tab, but I'm pretty sure that's always true.