WIP -- Value Def Customization UI

[ssharif6]

UI for customization of constant values (valuedef). Addresses issue #662.

[kanitw]

• [x] There is a bug in CompassQL for size value. (Putting quantitative X normally get ticks, but adding size value change ticks to points.)

Similarly, bars got changed to points too:

And line got changed to area:

In addition to fixing these, perhaps, it's worth trying more different combination of theses.

• [ ] The Value def UI is not styled yet. Better use consistent styles with fieldDef customizer. For example, text could look like axis title.

• [x] shape value customizer should be enabled only if the mark is point, text value customizer should be enabled only if the mark is text

• [x] For all of these values, there is no way to set "use default" value.

• [x] For text, if I type something and then later delete the text, then I get weird empty fieldDef.

• [ ] Size of point, text, and bar use different units (pixel area, font pixel, and width pixel -- we have to think about how we gonna make size work without causing problem to other marks.