Allow assigning categories for tracks

[tomasciccola]

In recent builds of comapeo - not yet published - we allow selecting a category for tracks (usually stuff like 'stream', 'road', 'limit', etc). This is basically done by setting the type of geometry for a category (point, line, area). I think this language is not necessarily the best for a user of the plugin, so a proposal to solve this would be:

1. Add a column in the categories sheet with smth like type
2. Default value is point (and/or p)
3. Another value we could allow for now would be track (or t)
4. In the future when we allow areas we could add area (or a)

Looking at the code, it seems that every category is assigned to every type of geometry which is good enough.
Since the version of comapeo that supports categories for tracks is not yet released, there is no urgency in implementing this (but ideally should be ready in the next couple of months)