ENH: User information can be saved to application settings

This PR supersedes https://github.com/Slicer/Slicer/pull/912

User information can be accessed through application logic:

userInfo = slicer.app.applicationLogic().GetUserInformation()

If the userInfo object is changed, the updated values are automatically written into Slicer.ini.

userInfo.SetName("John Doe")

Currently, validation is only performed for email address, just to illustrate how validation works.

userInfo.SetEmail("invalid email") userInfo.SetEmail("valid.email@somewhere.com")

Limitations:

• There is no API yet for deleting keys.
• There is no GUI for users to change their user information.
• No tests have been added yet.