Move the new build system to cdt.core and remove the previous
plugins. Hook the new system into scanner info and environment
variable manager.
Clean up API in preparation for Neon and API lockdown. Hook up
Qt to the new APIs.
Add discovery of MSYS2's toolchain and Qt and Qt's MinGW toolchain.
Change-Id: I85b1a91da4a44e86f0e9da9310f8106c894623e0
