add get/update from fn

[jrmoulton]

• Creates two new traits SignalRead and SignalWrite that just split out the accessing of the Rc data from the get and update traits.

• Adds a new signal type that lets you define a base signal and then a custom getter function and a custom setter function. This type implements the new signal traits

• Changes the checkbox widget to just accept in input that implements the signal traits.

• Changes the slider to use the more typical rusty new function instead of a create_*. (I'm leaning towards switching all of our APIs to this format).

• Adds a few different Slider::new_* functions that accept inputs that implement different sets of the Signal traits.

• Changes the slider example in the widget gallery to have the only state the the string for the text input and then the slider is driven by a GetUpdateFn with custom accessors.

I don't love the name GetUpdateFn but I haven't come up with anything better

[dzhou121]

For create_*, I personally prefer this over "RwSignal::new()`, because it blends well with all the view creation functions.

Also, it's simply easier to type in, I normally just put crrw and the auto completion will pick it up :)

[jrmoulton]

Part of the reason that I like the ::new_*() methods is because that works better for when there are multiple ways to constructor methods create a view. Like how I currently have it with slider. But I agree that in the simple case the create_* methods are cleaner. I'll just make sure to have a create_* function that creates the default view