Cross-platform conformity

This pull request contains changes to improve on the cross-platform capabilities of the WASABIQtGui project.
Changes contain:
- Addition of `c++11` to the qmake `CONFIG` variable (enables use of `std::to_string()`)
- Addition of the dependency libraries for mac operating systems (`LIBS += ...`)
- Replacement of `itoa()` calls with `std::to_string()` to ensure ISO-C++ conformity (see [this](http://www.cplusplus.com/reference/cstdlib/itoa/#portability))
- Fix of the OpenGL header inclusions for mac operating systems in glPADWidget.cpp

Depending on the compiler being used, the decision to use c++11 could render the current dependencies to *WASABIEngine* and *WASABI-qwt-clone* unlinkable. Compilation of the dependency projects in c++11 might become necessary.