Not really what you're looking for but it is possible to open and edit some libraries in the Arduino IDE. For example if the library is named FooLib:
- Create the file FooLib/FooLib.ino
- Create the file FooLib/.development
- Create a FooLib/library.properties file if it doesn't exist
- Restart the Arduino IDE if it's running
- Open the file FooLib/FooLib.ino
The reason for the .development file is that the IDE treats all files in a libraries folder as read-only unless that file is present. This feature was added a year ago so any IDE version older than that won't work.
This won't work if the library has subfolders like src. That hasn't been a problem for me because I put the source files in the root for backwards compatibility with IDE 1.0.x, though at this point there probably aren't many people using such old IDE versions. I use the FooLib.ino file for to-do lists and notes. I actually have my system set up so I can edit the keywords.txt, and README.md files in the IDE also as .ino files that are copied to the correct file names and the block comment operators removed from the first and last lines of the file and whenever they're modified.
The reason I do this is so I can have a consistent editor interface to work with instead of switching back and forth between two different applications. I like to use the Arduino IDE's editor so I can spot bugs and report them to the developers. I do prefer to use notepad++ for non-Arduino coding. The stupid automatic bracket feature they added and the broken code folding is making me seriously consider switching from using the IDE editor. I did recently get notepad++ set up to do Arduino style Auto Format using AStyle. After that the only thing I can think of missing from notepad++ is library keywords.txt handling which kind of sucks in the IDE anyway.