Link URI re: issue #486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
I was translating the new strings on transifex and testing this new function. While "file: links" are prevented, this does not forbid to enter local paths like D:\My Documents\myfile.txt or /home/USERNAME/Documents etc... Aren't they much more common than "file:" ?