Don't unassign non-matching OpenPGP keys

If a contact creates a new signing subkey which is not yet known to us,
Psi shouldn't unassign the master key.
Group similar actions in the contact menu, add missing shortcuts

Grouping similar actions reduces the menu's complexity, thus
increasing usability.
Add icons to grouped contact menu entries

New icon: psi/manageContact
Don't change the type of incoming error messages
Don't send receipts for returned error messages
Ignore more error message payload
Merge pull request #145 from FlorianFieber/fix-error-type

Don't change error type messages or handle their payload
Merge pull request #144 from FlorianFieber/usability

A clearer context menu for contacts
Merge pull request #141 from FlorianFieber/no_key_unassigning

Don't unassign non-matching OpenPGP keys
Auto-assign known PGP keys when receiving signed presences

Already assigned keys will not be overwritten.
Merge pull request #146 from FlorianFieber/auto-assign-known-keys

Auto-assign known PGP keys when receiving signed presences