Fix/update project building

Modernise the project building.

• Add require checks for all enabled optional dependencies
• Add error messages for build options that require other build options
• Remove Python 2 support as this is EOL
• Replace detection of Python and gi module with meson's find_installation().
• Add project_url variable to build files.
• Add project_api_version variable to build files.
• Add license string to build file.
• Delete gexiv2.pc.in file as this is not used by the build. Meson creates the file dynamically.