Adding support to search in Visual Studio solutions

[ldufr]

The main goal would be to use the .sln solution files for Visual Studio and search all files included in projects of this solution.

Some stuff to consider:

• We would want to support multiple solution files.
• We would want to reload the list of file if they are updated on disk.
• If we parse every file containing ".sln" as a Visual Studio solution through the existing "paths" input, we can't search in those file anymore. An option is to use a "protocol" approach, a bit like file:// or http://. In this case, it could be vssln://. An other option would be to add a checkbox to the side and allow a user to have multiple "paths" input.