fully configurable variable menu

[hotzeplotz]

in Pattrn v2.0, the list of variables has been moved from a popup menu (Pattrn v1.0) to a side panel, as we now support an arbitrary (and potentially large) number of variables and filters rather than the 15 max of Pattrn v1.0. Moreover, as an arbitrary number of layer groups can be used (#30, pending fixing of several bugs related to this new feature), the list of variables available on a Pattrn instance can now be very long.

However, these long lists can be hard to navigate, especially as the order of layer groups, layers and variables is generated automatically from the order of variable types and variables defined in each layer group's metadata.

In Pattrn v1.0 variables were grouped by variable type, with a submenu of the main filter popup menu being used for each variable type. This grouping could be reproduced in Pattrn v2.0: perhaps combined with an accordion visual layout, leaving variables within a group hidden by default, except those with an active filter applied.

However, it is likely that in several contexts variables of a different type are actually analytically related, making a simple per-variable type grouping less useful.

A fully configurable menu layout should be implemented to improve Pattrn's UX, directly mapping the analytical aims of each instance to the layout of layer groups, layers and variables lists.