filter: implement saving of filter presets to git repositories

On the mailing list it was decided that users might want to
move their filter presets across computers via the cloud.
Notably, in the future one might access statistics on mobile and
these might by controlled by filter presets.

The git save routines use the same string formatting as the
XML save routines. The string formatting is found in
core/filterconstraint.cpp. Thus, duplication of code and
inconsistencies should be minimized.

Each filter preset is saved into a file in the "02-Filterpresets"
folder in the root of the git repository.

Each file consists of one "name" line, zero or one "fulltext" line
and zero or more "constraint" lines.

The modes, types and the actual payload is controlled via attributes.
Thus, a preset file might look like this:

name "test"
fulltext mode="substring" query="clown"
constraint type="location" stringmode="starstwith" data="mafia"
constraint type="sac" rangemode="range" negate data="5000,10000"

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

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