Meta: improve code quality

[federicobond]

This is a meta issue to keep track of changes needed to improve code quality:

• Fix build with Qt5
• Fix most warnings thrown by the GCC compiler
• Configure CI build system for building in Linux (bonus: OS X and AppVeyor for Windows)
• Fix tests build

Nice to have:

• Use clang-format to maintain a consistent code style across the codebase
• Use clang-tidy to update code with modern standards
• Fix compilation with Clang
• Move build system to CMake