application: Use better keyboard accelerators

Use "<Primary>+n" to add a new contact and "<Primary>t" to open a
transient identity window.
tox: Improve debug message

Append ellipses to the debug message in neuland_tox_send if we cut the
message off.
tox: Clarify calculations in neuland_tox_send

These changes makes it easier to understand the calculations for
splitting up long messages and fix one small issue in them, where we
sent one byte less than possible.