emit view->events.destroy on xwayland unmap

Created by: Ongy

Emits the view destroy signal when an xwayland view is unmaped. This makes sure, no references to it are held, and fixes a few situations where inconsistent view state could lead to segfault.

This fixes the segfaults, but I am honestly not sure, if it could introduce other issues. Quick testing doesn't expose anything obvious, but there may be more complicated issues.

Relevant issues: https://github.com/swaywm/wlroots/issues/705 https://github.com/swaywm/wlroots/issues/679 https://github.com/swaywm/wlroots/issues/671 https://github.com/swaywm/wlroots/issues/650

The view_finish in desktop.c uses the same libwayland signal emit. Should we change that (in another PR)?