We have to call close_view(), which calls IPlugView::removed()
before destroying the parent widget.
Previously ~VST3PluginUI() was called after the derived class
destroyed the owned private `_gui_widget`, `_ns_view` or x11 parent.
On windows that may lead to the hwnd of the window becoming invalid
before the call to ::removed().
In theory the GUI is single threaded and registration of
events happens only when a plugin GUI is constructed or
destroyed (which happens in the GUI thread).
Except it seems that if a plugin uses reference counting,
un-registration can happen later.
Some plugins correct their own size when resized which can lead
to endlessly growing GUIs. In particular this issue exists
if Ardour's plugin-toolbar forces a plugin to grow in size to fill
the allocated space. While the plugin aims for fixed aspect-ratio.
This will need a proper solution involving VST's checkSizeConstraint,
however GTK+2 does not have an appropriate API...
