[PATCH] Validate GPX

[hampelratte]

I have created a patch, which enables validation of GPX files, when they are imported. If an error occurs, a JOptionPane pops up with details from the parser (line, column and error msg).

This shall encourage developers of other software to make their software GPX compliant and inform JOSM users, that it's not JOSM fault, that a file can't be read.