I'm not a fan of Hungarian notation, but C++ kind of needs it with its
implicit `this`. And of all the common options for this, I find
suffixing members with an underscore the least offensive one.
This reverts commit a495f1a67f.
This ended up not being an issue. What we _do_ have to do, sadly, is to
have a mutual recursion context stack per plugin. Otherwise multiple
plugin instances can deadlock eachother.
This has much fewer moving parts, and it's probably more understandable.
There was also a race condition in the previous implementation, so
there's that.
This requires a super hacky workaround because the UI thread can be
currently blocked by the plugin calling `IPlugFrame::resizeView()` from
the Win32 message loop.