This touches a lot of components, so splitting things up would not allow
us to be in a buildable state.

In particular:

 - IdeBuildSystem made an interface
 - IdeProjectMiner made an interface
 - Autotools plugin created, with build system and miner
 - Fallback plugin created, with directory fallback build system
 - Context updated to take this into account
 - IdeObject can now be used to create build systems from plugins

We should follow this up by moving version control to plugins as well.