new input control: checkbox

[go-oleg]

Add a new input control type called checkbox. It should be rendered as an <input type="checkbox/> and return a boolean value.

Example

input myBooleanValue: checkbox -label "Enable downsampling";

Open question:

• Each input currently has a -label option which is currently rendered above the actual input contents. That probably isn't going to look good with the checkbox which should have a label next to it. Should play around and see what works the best w.r.t. to maintaining a consistent look across inputs.