Allow debugger dock to float

[ryevdokimov]

#113133 added the debugger to the new dock system but disabled the ability to float due to the debugger being set to nullptr on exiting the tree, which happens during a reparent (which happens when making the dock float).

I don't really see the purpose of _debugger_gone_away(). There isn't a way to remove it under any normal conditions that I can find, and if something were to be implemented for some reason, it probably shouldn't be done via the tree exiting due to the things mentioned above that trigger tree_exited.

bugsquad edit:

• Fixes: Error "Plugin is not attached to debugger" when alt-tabbing the editor after placing the Debugger into a most bottom dock #117243

[KoBeWi]

The Debugger dock could be now global (remove set_global(false)).

[KoBeWi]

_debugger_gone_away() was added in #66938
CC @Faless

[kitbdev]

Tested, works as expected.
Code looks good, I don't see why _debugger_gone_away would be needed, it gets cleaned up on the destructor. Reparenting the debugger while it is running doesn't cause issues either.

It is currently possible to move the debugger to the additional bottom docks which causes Plugin is not attached to debugger. errors, so this has become a bugfix.

[Repiteo]

Thanks!

[Naros]

@KoBeWi it seems in 4.7-dev4 on Linux, this PR causes the editor to crash when closed/project reloaded. Reverting this PR change and the editor works again. I haven't looked into it, but thought I'd report its not working as expected.

[KoBeWi]

Can't reproduce on Windows. Open a new issue.

[Naros]

@KoBeWi done, #118518