Creating install for C++ version

[dpizetta]

Providing setup files for popular c++ build systems (qmake, qbs, cmake...) that build a shared/static library with the stylesheet resource and a helper class that helps loading the stylesheet.

I set it to the milestone of version >= 3 that is in the branch dev. This is just to get it tracked.

Discussion in PR #98 .

[suratovvlad]

Hello! I've created a C++ example using this stylesheet in my libqdark library.

However, I was forced to modify *.ui files to change the class names (here is the patch), because it breaks One Definition Rule.

I'm using this custom library in several projects keepassx-dark and texstudio-dark. I didn't tested with Windows and MacOS yet, however it shouldn't be hard to fix compability

[gusmd]

Just wanted to add that I've used QDarkStyleSheet on a simple C++/CMake project as well, pubfix.

The way I included QDarkStyleSheet was to simply clone this repo as a submodule, and directly add the qrc file to my add_executable call on CMake. See my PR. It's less elegant than having it as a library, but avoids issues with Qt not loading the resource because it is in a separate library.