[Tracker] Extensions clean up

[Martyn Russell <martyn lanedo com>]

Hi all,

I want to clean up the extensions/ directory. Right now there are only 2 
sub-directories, one for firefox and one for thunderbird.

As much as I would like to have support for these in Tracker, the code 
looks quite old and would need someone to take it on. Is anyone 
interested? Also, if we decide to keep it and not remove it, I will move 
it to src/plugins/.

Thoughts?

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Regards,
Martyn