[wxMac] fix for app.cpp when using wx in embedded situations

[Stefan Csomor]

Hi

all who are using wx in an embedded situation and unload wx before the
host app terminates might have had sporadic bug reports ending in
something like

Thread 0 Crashed:
0   mywxcontaining.dylib                 	0x113f0160
macPostedEventCallback + 28
1   com.apple.CoreFoundation       	0x907dd4cc __CFRunLoopDoSources0
+ 384
2   com.apple.CoreFoundation       	0x907dc9fc __CFRunLoopRun + 452
3   com.apple.CoreFoundation       	0x907dc47c CFRunLoopRunSpecific
+ 268
4   com.apple.HIToolbox            	0x93208740
RunCurrentEventLoopInMode + 264

after wx has already been unloaded

this comes from an incomplete cleanup in app.cpp of a CFRunLoopSource,
I've fixed app.cpp in CVS HEAD and WX_2_6_BRANCH, so please update
app.cpp in your code, test and report if this somehow causes problems

Thanks to Richard Oehlinger for sending me the crashlog

Stefan