CMake: Support imagemagick, drop manpage compression, quoting, VERBATIM

 * Support imagmagick's convert as fallback if rsvg-convert is not available.
 * Drop manual page compression.
 * Quote all strings.
 * Use VERBATIM for all custom commands.
 * Use additional (version independent) vala/vapi vapidir for valadoc
Use absolute path to save recent project files.
WelcomeScreen: Use ~ as abbreviation for home directory.