Allow proving, editing and compiling as different commands in cement

[jklmnn]

Currently cement only supports xml and gpr files. For the first case it builds a complete system and for the latter it opens GPS with the project.
For proving a gpr file is also needed. To select between editing and proving a subcommand should be added:

• build, compile system, requires xml file
• edit, edit project in GPS
• prove, prove project with gnatprove

[jklmnn]

Fixed by #173.