Check for triggers and foreign keys that could prevent rebase

[wonder-sk]

Currently users do not get any warnings if they have some custom triggers or foreign keys in they GeoPackages, yet this is not supported by geodiff when doing rebase - causing creation of conflict files and possibly more problems coming from conflict files.

The plugin should therefore check for these during validation.

Maybe at some point there will be API in geodiff for this check - MerginMaps/geodiff#49 - but in the meanwhile it won't hurt to just implement similar code in the plugin.