Sqlite/Pythons detection what to convert to a string seems not reliable,
so always return bytes and convert all TEXT columns

- Dont use cursor object
- Use namedtuple factory

- Merge all stores into AxolotlStore

- Adapt to nbxmpp supporting OMEMO
- Move python-axolotl code into backend folder

- Adapt to nbxmpp now supporting openpgp

- Use IconTheme
- Add/Remove some clients

Fixes #377