Use a semitransparent black shadow for embiggened tray icons

On X11, Psi previously added an opaque gray "shadow" to tray icons which
looks out of place on dark or colored panels. A semitransparent black
shadow should look cromulent on any panel color.
Merge pull request #137 from FlorianFieber/tray_icon_shadow

Use a semitransparent black shadow for embiggened tray icons
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.
Replace PGP icon with a less pixelated one
Merge pull request #142 from FlorianFieber/icons

Replace PGP icon with a less pixelated one
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
Out of source builds fixed
Put Psi binary to build dir instead of source dir
Mark messages as handled after they are processed.
Request notification for messages send from chat dialogs if stream management is active.
Show a throbber in chat dialog when there are messages unacked.