Escape the file name

Since we interpolate this in XML and use .set_markup)(), we have to
escape '&' and '<'. This fixes broken display of file names with
these characters.
Better exception logging
Generate a directory name that mentions Virtaal
Add missing import
Move/remove imports
Eliminate import of logging for another case

This will avoid the import of logging for another case: when virtaal
is started with just a filename and no other command line options.
Update the required GTK version
Give a hint about disabling dependency checking
Simplify code for printing already imported modules
Drop Python 2.5 from Travis builds
Support Google Translate version 2 API
Get the supported languages from Google
Complete change from 2d7e13eb