[NifSkope.pro] Qt version checking, MSVC improvements

Explicitly require Qt 5.3 or later. Bumped up
QT_DISABLE_DEPRECATED_BEFORE to 5.3.

Greatly improved MSVC detection regarding distinguishing between Qt
Creator and Visual Studio.

Properly enforced a minimum Visual Studio version requirement.  Did not
account for later VS versions like 2014, which would be falsely flagged
as unsupported.  Instead does a numerical comparison to the _MSC_VER
defined in Qt's mkspecs.

Much saner VS vs Qt Creator checking overall by limiting the scope to
Windows/MSVC.  Most code will be correctly ignored on MinGW/GCC.