Renamed MixxxApp to MixxxMainWindow.

This is required, because we need to inherit a new class from QtApplication
later. A MixxxApp Inherited from QMainWindow is confusing.
Removed unused crashDlg()
Made loadTranslations a private member of MixxMainWindow; Fix some MixxxApp left overs.
Moved MixxxMainWindow Constructor and destructor to the top of mixxx.cpp
Some coding style fixes
wslidercomposed: made some functions protected and added virtual keyword