Add DerivedDrawHandle

[dhardy]

The intention here is to allow themes like ShadedTheme to be incomplete by providing some base implementation to fill in missing methods. Currently ShadedTheme does effectively the same thing but with stub functions; part of the point of this change is to make it non-breaking (or less breaking) to extend DrawHandle and SizeHandle.

Blocker:

error[E0119]: conflicting implementations of trait `draw::handle::DrawHandle` for type `std::boxed::Box<_>`
   --> src/draw/handle.rs:737:1
    |
568 | impl<D: DerivedDrawHandle> DrawHandle for D {
    | ------------------------------------------- first implementation here
...
737 | impl<H: DrawHandle> DrawHandle for Box<H> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `std::boxed::Box<_>`
    |
    = note: downstream crates may implement trait `draw::handle::DerivedDrawHandle` for type `std::boxed::Box<_>`

Fixing this would need some form of specialisation or negative trait bounds (e.g. rust-lang/rfcs/issues/1834).

An alternative would be to add the base* methods and default implementations to DrawHandle itself; this is less desirable since themes without a base (e.g. FlatTheme) would be forced to panic in the base* methods, resulting in run-time failure if the theme is incomplete (and only then on examples which use those size/draw methods).

Incomplete in this PR:

• switch ShadedTheme to impl DerivedDrawHandle
• add DerivedSizeHandle

[dhardy]

This is made obsolete by #327, though is potentially another alternative.