Building the IDE without bundling the toolchain

[As discussed on the mailinglist](https://groups.google.com/a/arduino.cc/d/msg/developers/4jL4nL_YJ1k/7ZvQhwP3Qy8J), here's some commits to allow building the IDE without bundling the toolchain, to allow advanced users to use their system toolchain. It's probably not finished yet, I'll comment in more detail on the mailing list.