Add editor setting for AcceptDialog OK/Cancel buttons positioning

[akien-mga]

The position (left/right) of the Cancel and OK buttons in AcceptDialog
are DisplayServer specific, as Windows uses OK/Cancel and macOS uses Cancel/OK.

Linux/X11 currently uses the macOS convention which is also the GTK+/GNOME one,
though it's not consistent with Qt/KDE applications which follow the Windows
convention.

Since that can't satisfy everyone, it's best if it's configurable also for the
editor (it's already configurable for the project).

Fixes #59379.

@bruvzg I used "OK First" instead of "OK Left" assuming that this would be handled with UI mirroring too for RTL, but it doesn't seem to be (at least from a quick look at scene/gui/dialogs.cpp). I don't know if it should be, we'd have to check e.g. Arabic on Windows and macOS.

Also, I find this whole swap_cancel_ok API in AcceptDialog a bit hacky (and badly named as it doesn't tell users swapping from what to what, i.e. that the "reference" unswapped positions are the GNOME/macOS ones). And on Linux one could make a case to have the behavior DE-dependent so that on predominantly Qt-based KDE it would follow the Windows convention... but that's a mess (and as a KDE user I wouldn't like the behavior to change now that I'm used to OK on the right).

[bruvzg]

I used "OK First" instead of "OK Left" assuming that this would be handled with UI mirroring too for RTL, but it doesn't seem to be (at least from a quick look at scene/gui/dialogs.cpp). I don't know if it should be, we'd have to check e.g. Arabic on Windows and macOS.

There's no need for extra code in the scene/gui/dialogs.cpp, RTL layout will mirror the underlying box container.

[akien-mga]

As discussed in PR review meeting, I'll update this to also change the project setting to use an enum like done here for the editor setting.

Might also research whether we should auto-detect the preferred style on X11.

[akien-mga]

I'm not finding time to rework this and handle the project setting, which would turn out to be a big rabbit hole as there's a lot of inconsistencies in our handling of OK/Cancel to handle to do this right.

So I think this could be merged as it improves the situation for the editor somewhat, which is the main Godot project where the position of such buttons matters and might need to be customized by the user.

[timothyqiu]

Cherry-picked for 3.6.