CMake handling library

[Manu343726]

This PR adds a C++ API for loading and building CMake projects and targets. It also fixes some bugs in the signals API.

The features of the siplasplas-cmake library are:

• Load CMake targets metadata from JSON file.
• Configure and build cmake projects.
• Automatically rebuild cmake targets on source changes.
• Automatically reload target output binary (library) after target rebuild.

All watching and building is done asynchronously if triggered by source changes, the API uses siplasplas-fswatch (A wrapper of efsw using asynchronous siplasplas-signals for watcher thread -> main thread message passing).

[Manu343726]

Ok, I'm going to merge it, but the project is in an unmaintainable state (no tests, no docs, ci builds failing) that cannot be ignored (more...), so as soon as the label becomes purple I would be opening a branch to handle the issues.