Split wxWakeUpPipe class in MT-unsafe and MT-safe parts.

Split wxWakeUpPipe class in MT-unsafe and MT-safe parts.

This class can also be useful when it's used in the main thread only, so
leave the lock-less part of the code in wxWakeUpPipe and derive a separate
wxWakeUpPipeMT from it for the use in wxConsoleEventLoop where it can be used
by multiple threads.

See #10258.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74339 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775