Handle startup in presence of old settings files without crashing

[mborgerson]

We seem to have an issue with loading settings files created with an older version of the tool. XQEMU-Manager should be more robust in loading these older files instead of crashing.

[JayFoxRox]

We should redesign the settings class altogether.
It should also be able to handle platform-specific default values for example.

While working on this, we should probably also clean up the existing JSON layout (which often uses different terminology or different naming schemes). The current layout also uses wrong data-types in some cases.