the rest from `tools/convert_boost.sh`.
* replace boost::function, boost::bind with std::function and std::bind.
This required some manual fixes, notably std::placeholders,
some static_casts<>, and boost::function::clear -> = {}.
This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.
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().
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...
