"clear all" button, readily available

[thorwhalen]

When some functions have a lot of arguments, or if we implement a "sticky mode" (where previously entered values are cached and used as defaults) it may be good to have a "clear all" button (including or not (option?) default values).

[thorwhalen]

Still open: no "clear all" button/action found in `front/elements/elements.py` or the CRUDE dispatch code. Depends on the "sticky mode" (cached previous entries as defaults) this issue mentions as one motivation, which per #10 also isn't fully built yet (the state/binding substrate exists, but not a resolution policy that would make values "sticky" across runs by default). Leaving open.