`cmake --install` is broken

[tronical]

There are two issues when running cmake --install $builddir:

1. The application binary isn't installed.
2. When Slint is built via FetchContent, Slint is installed with headers, slint-compiler, etc. instead.

Expected outcome:

1. The application binary is installed
   2.libslint_cpp.so is installed next to it. Perhaps this could be done using CMake's runtime dependency support.