Sort by metadata

[amir-zeldes]

The addition of sorting to the query results is great (though it might be good to give it a more prominent button in the results panel itself, as many users are not aware of it). It would be very useful to also allow sorting by metadata, using:

1. The extended search options - a text box (or drop down for selected corpora) could be offered where a metadatum could be input. If a non-existent metadatum is selected a warning should be used, but results can still be displayed (using default order). If only some of the selected corpora/documents have the metadatum, results from those should be displayed first, in order, and then there rest (where the metadatum is 'NULL' or non-existent) should be output in default order afterwards.
2. From the meta-data button of each search result (the i-button) it would be possible to add a button for each metadatum in the metadata panel to "sort current results by this metadatum".
3. From corpus.properties a default metadatum sorting preference could be set for specific corpora (always start results as sorted by X for this corpus). This makes ANNIS less dependent on document names, which may be meaningless or have a tricky sort order.
4. Conceptually, this should also be expressible in the query builder (though it is generally lagging behind in terms of expressing advanced options, so this is not necessarily the first thing to do)