Store tags in Thunderbird custom fields

Store tags in Thunderbird custom fields

This adds a new config option called tbtaghack. When enabled the first four
assigned tags will be put into the fields custom1 to custom4 which are read
Thunderbird's addressbook.

Note those tags are writeonly, if you change a custom filed through other
applications, ConTagged will not know about it. Tags are stored in the
marker attribute additionally - only those are used inside ConTagged.

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