Qt headers produce a lot of -Wconversion warnings

There is no warnings for implicit conversions between reals and integer by default with GCC. Maybe this is not a real issue, but when we enable those warnings with -Wconversion flag in our .pro file, we also have warnings from Qt headers.

Normally we use -isystem instead of -I for adding include path of our dependencies to avoid warnings from 3rd party library, but with Qt we can't do that.