Add ability to drag-toggle visibility in scene tree editor

[mihe]

Resolves godotengine/godot-proposals#5488.
Supersedes #102009.

This adds the ability to drag-toggle the visibility of nodes in the scene node editor, where the toggled state of the first node is applied to any subsequent node whose visibility button you drag across.

Unlike #102009 this involves no Tree changes (which seemed to be a point of contention there) and is instead contained almost entirely within the gui_input handler of SceneTreeEditor.

visibility-dragging.webm

A couple of noteworthy quirks:

1. The undo action is created when you release the mouse button, meaning after we've already changed the visibility states. There may potentially be some edge-cases where we never actually put the changes on the undo stack.
2. Since we don't suppress the Tree button handling when starting the drag operation, the visibility button you start from will be in a pressed state for the duration of the drag.

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Disclaimer: AI tooling was used to review this change, which prompted a few minor changes, all of which have been carefully reviewed to the best of my ability.

[kleonc]

Seems the drag is not being interrupted/reset on focus lost, this can lead to incorrect state. To see it in action
(tested on Windows 10):

1. Start drag-toggling the visibility buttons by LMB press.
2. Alt+Tab to another app.
3. LMB release.
4. Alt+Tab back to Godot.
5. Hover the mouse over the visibility buttons, it's still in drag-toggling state (with mouse not pressed anymore).

[KoBeWi]

On that note, maybe there should be a way to cancel the drag? Like RMB or Escape.

[mihe]

Seems the drag is not being interrupted/reset on focus lost, this can lead to incorrect state.

Fixed (?) by resetting the drag state on NOTIFICATION_APPLICATION_FOCUS_OUT/NOTIFICATION_WM_WINDOW_FOCUS_OUT.

On that note, maybe there should be a way to cancel the drag? Like RMB or Escape.

I assume ui_cancel is meant to do that kind of thing, and seems to be what Viewport uses to stop its dragging as well, so I just copied what it was doing. Viewport does however not respect ui_cancel when bound to a mouse button, which seems like it might be a bug, but I copied it anyway.

[mihe]

On that note, maybe there should be a way to cancel the drag? Like RMB or Escape.

It turned out that Viewport was in fact stopping its dragging on RMB as well (which perhaps was why you suggested it) so I threw that in as well.

[passivestar]

Tested, works as expected. A much needed UX improvement.

[Calinou]

Tested locally, it works as expected (including undo/redo and crossing gaps of nodes without visibility toggles). Code looks good to me.

[keestak]

This pr looks good, I included the extra api for a generic click and drag in my own pr because it was requested in the issue I linked and this doesn't, but it's probably cleaner as a result. I'll close my pr since this one supersedes it.