Run certain GUI tasks from the host's run loop

This was a bit of a tricky one because it requires simulating mutual
recursion, but it's needed for REAPER as otherwide calls to
`IPlugFrame::resizeView()` and `IContextMenu::popup()` might cause
REAPER to segfault because its GUI is not thread safe.

meson.build

@@ -454,6 +454,7 @@ if with_vst3
       boost_dep,
       boost_filesystem_dep,
       bitsery_dep,
+      function2_dep,
       threads_dep,
       tomlplusplus_dep,
       vst3_sdk_native_dep,