refactor dialog_canvas.html to use javascript objects instead of ["name1", "value1", "name2", "value2", etc.] arrays

change fielddesc_getfloat to fielddesc_getcoord so that the (admittedly arcane) scaling/quanta syntax can be used with [draw] methods where it makes sense

do a proper loadbang for canvas fields (needs more testing)

(differs slightly from mine in that the '-' character is used to mean
 a wildcard template)

from pd-l2ork: 9e88d7ac62c7c4a2717efe7953ac2165e1c5b816

from pd-l2ork: fb109233c5b8c2ed4bbc5d0f7598cff4e2ed7a10