Installing a callback now returns a CallbackHandle. All map-style APIs
install this handle automatically on the created dynamic, which keeps
the callback installed until the dynamic is freed. All other APIs
return the handle for the caller to either call persist() or store
somewhere.
Now, the dynamic system can be used for application-long data with
almost no fear of leaking data due to how callbacks are being installed.
Technically cycles are still possible by moving clones into the
callbacks, so a WeakDynamic type might be worth exposing.
Introducing two new colors:
- ColorTheme::color_dim, for dimmed/disabled primary colors
- SurfaceTheme::opaque_widget, for buttons.
In material design, a button's background color uses the Highest
Container role, which seems incorrect because then buttons wouldn't have
a different color when placed inside of the highest level container.
Rather than remove a container level, I added one more tone using the
neutral variant.
Other changes are just gut feelings to have a slightly richer dark
theme. I feel like material is a little muddy in dark mode.
