[libcontentaction] Force usage of invoker and add X-Nemo- keys

This allows third party developers to just use a standard .desktop file
with Exec= lines, while still having all the benefits of boosters and
mapplauncherd's invoker. Single-instance is also enabled by default,
but can be disabled via a special "X-Nemo-Single-Instance=no" key.
Merge pull request #5 from thp/force-invoker-usage

[libcontentaction] Force usage of invoker and add X-Nemo- keys