It is its own project that can have its own release schedule.  This way if patches need to be made, they can without waiting for a core release.

Currently the webcontrols are still using a 1.x codebase, to support modules on this framework version.  Obviously, the controls will still work in 2.x of the framework.  You should be fine adding the project to you solution.  However, if you ever wish to patch your webcontrols this may not be as easy if you do so.  Right now a patch can be applied by simply inserting the dll into the bin folder and if you plan on recompiling, the DotNetNuke\Library\Controls\DotNetNuke.WebControls\bin folder (where it is referenced by the core).