Find suitable descriptive names for different collection types

[sunkup]

As of right now we have the naming conventions of

• "local collections" for collections storing the actual contents in the content providers. Thats is: LocalCollection, LocalCalendar, LocalJtxCollection, LocalTaskList, LocalAddressBook
• "DB collections" for representing remote collections stored in the database.
• ... the actual remote collection saved on the server

The naming is ambigous and somehow confusing. We should think of some new names, rename the classes and update the KDoc.