[OBO MykoMap] Support explicit configurabilty of field used in Directory panel

[lin-d-hop]

Please track against the clockify project [OBO MykoMap]

Description

Within MM the Directory panel is an expanded version of the first filter in the Search panel. In this issue we want to make it possible to specify another filter, besides the first one, as the filter to be expanded in the Directory panel. The goal is the create a new field that allows any of the filters from the Search panel to be selected, which can be specified in the MM config file.

Acceptance Criteria

• Create an addition MM configuration option enabling a specified filter to be selected. This could be done by specifying an integer or text string, ideally readable yet flexible for any case.
• If this configuration is missing or throws an error, the default should be the first specified filter as now
• Ensure backwards compatibility with all MM (hopefully handled by the above)
• Ensure the case that no filters are defined is handled. Possibly by also not showing the directory. Or whatever happens now (ie backwards compatibility).

[lin-d-hop]

Replaces #237

[wu-lee]

An extra detail: the directory panel pre-dates the search/filter panel, and as such has its own separate implementation for finding the categories, which duplicates the filter functionality in a different way. (In brief: an index is built up when the data is loaded and stored as a look-up of look-ups.)

My intention for a while has been to remove that duplication and use the filter implementation. This is a separate task, but which perhaps should be done as part of this one (or prior to this one).

One consequence of this duplication of functionality is that the directory doesn't really play well with the search history management, see #205 (comment)

[wu-lee]

In line with the way the config is going right now, I suggest that this is configured with a flag in one of the properties of the propDefs (i.e. fields) schema of the config. The filterableFields config value which used to define the list of filters - and so which one was in the directory - is partially deprecated already as a result of #205.