DolphinQt: add Resource system to load themes.

I've also disabled QT_NO_CAST_FROM_ASCII, as leaving it enabled
would require adding "QStringLiteral()" around every single string.
The only benefit that you get from this is having the ASCII->UTF
conversion happen at compile time. Unfortunately, the C++11 feature
that does this is not supported in VS13, so it just falls back
to a runtime conversion anyway.

(I've left QT_NO_CAST_TO_ASCII enabled.)