Split elements getAssets method into listAssets and getAssets

The static method listAssets must list all assets required by the
element, while the non-static method getAssets can return a sublist,
depending on configuration for example.
Refactored ApplicationAssetAction to use asset caching
Added MapbenderAssetDumpCommand
Merge branch 'feature/sass-restruct' of github.com:mapbender/mapbender into feature/sass-restruct