[GTK] closing event working as documented

[p4bl0-]

This is what I view as a complete fix for the closing event beahvior under GTK. I did a separate pull request from a smaller fix because I'm not sure this one is wanted: maybe the documentation is wrong and the code was right.

What this does is to actually run the events.closing handlers before the closing confirmation (if confirm_close is set), as explained in the documentation (see issue #1178).

It also simplifies the GTK code by merging the close_window and on_destroy functions, in the spirit of how it is done in the Qt code.

Maybe the new merged function should rather be called on_window_close to match other event handlers. But that's not really for me to decide :).

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As a bonus, it also looks up the value of the confirm_close property on the pywebview window at event time rather than initialization time, so that this can be dynamically changed (see the first comment of issue #1178).

This new behavior should be backward compatible with the previous one except if some code exists that changed the value of the confirm_close property of their pywebview windows with no effect previously (but why would anyone do that?).

However, this new behavior will differ on other backend as is, as I could only test it with GTK so that's the only one I changed in this PR. But note that changing this behavior for other backends as well should be very straightforward since they all have access to the pywebview window.

[p4bl0-]

As a bonus, it also looks up the value of the confirm_close property on the pywebview window at event time rather than initialization time, so that this can be dynamically changed (see the first comment of issue #1178).

This new behavior should be backward compatible with the previous one except if some code exists that changed the value of the confirm_close property of their pywebview windows with no effect previously (but why would anyone do that?).

However, this new behavior will differ on other backend as is, as I could only test it with GTK so that's the only one I changed in this PR. But note that changing this behavior for other backends as well should be very straightforward since they all have access to the pywebview window.

This was actually already the behavior in the Cocoa and Windows implementations!

See here for windows, and here and there for Cocoa.

[p4bl0-]

What this does is to actually run the events.closing handlers before the closing confirmation (if confirm_close is set), as explained in the documentation (see issue #1178).

As tagged in the title, this PR only fixes the GTK backend.

Please anyone with either a Windows, a Mac, or a Linux with a working Qt backend test #1181 so I can mark it as ready :).