Make it possible to edit libraries in the IDE

Books on Arduino give the advice that "Because the Arduino IDE is set up to recognize .ino files, it is often easier to write the header and source code files using a simple text editor like Notepad" ([Jack Purdum, Beginning C for Arduino, Second Edition](http://www.springer.com/us/book/9781484209417)).

This is a shame.
The Arduino IDE should be able to open libraries for editing in a sane way.

Start by examining how the Arduino IDE currently opens sketches and discovers libraries; the issue provides no file or test entry point. Done means users can open and edit library header and source files from the IDE in a sane, documented workflow.

Written by the indexing model from the issue text.