Better configurations sharing between Linux and Windows

[InSyncWithFoo]

Feature or enhancement

On Linux:

• Virtual environment executables directory: venv/bin/
• Extension: None

On Windows:

• Virtual environment executables directory: venv\Scripts\
• Extension: .exe

It would be nice if project configurations can be shared among colleagues who use different OSes.

Proposed solution

A new configuration should be added as a checkbox in the project-level panel. The path resolving algorithm also needs modifications:

1. If there is a file/directory/symlink/etc. at the precise path given, use that.
2. Otherwise, if the topmost fragment has no extension, append the corresponding extension by OS and redo step 1.

The hard part of this is to make the UI hint react when the checkbox changes its state.

[InSyncWithFoo]

Fixed by #79.