[obexd] Filter files to be received programmatically. Contributes to JB#16243.

Tiny plugin for checking whether incoming files should be received or not.
Dummy plugin accepts all files; helperapp plugin calls an external helper
to make the decision. The helper can be implemented e.g. with libcontentaction
to determine if there's a handler for the file in the system.
Merge pull request #16 from hmallat/OPP-reject-unsupported

[obexd] Filter files to be received programmatically. Contributes to JB#...