Feat/valid optimisation

[Guillaumebeuzeboc]

Bring optimisations to the Gazebo snap. The optimisations concern the size of the snap as well as the size of the installation in /snap/gazebo/current.

The following optimisations have been done:

• Remove every file already present in a content sharing
• Remove unnecessary files (headers, sources, cmake files, examples, man files, perl doc)
• Remove unnecessary Debian packages only necessary at build-time

This results in the snap being 48% smaller and taking 44% less space on the disk once installed.

[artivis]

Is it really necessary to have the startup-time.sh script here? We agreed on leaving a branch with all the other optimizations attempts, let's keep the script there as well.