Fix wxCommandEvent/wxThreadEvent subclass mismatch

Regression by 3fed975bac11956ffc4d3eae86c928c7e0c921db caused netplay to
crash on OS X.  While I'm at it, fix the long-standing "unsafe i guess"
AddPendingEvent, since we depend on wx 3 now...
Merge pull request #1558 from comex/command-thread-mismatch

Fix wxCommandEvent/wxThreadEvent subclass mismatch