UI: In parameter mode, edit only the value, not key or assignment symbol

[frothga]

This will require a more sophisticated editor panel that contains both a label (icon + text) and a text field. Multi-line editing would not be permitted. The construction of the editor panel can take advantage of column widgets developed for issue #100.

Allow user to specify what editing component to use from a limited selection (text field, slider, spinner, drop-down list). These can be specified in the "param" metadata tag, mostly be specifying the kind of data expected, including ranges and a fixed set of choices. Examples:
param=0 -- Reserved to set param to false. Any other value is true, and interpreted to indicate widget and range of choices.
param=A,B,C -- A drop-down list with three options
param=[min:max:step]{units} -- A spinner or slider. Optional units will be appended to the number to form the final value inserted into the parameter.