Remove return values on some closures; replace some custom widgets

[dhardy]

fn AppData::handle_messages previously returned Action, however:

• The only sensible return values are Action::empty() and Action::UPDATE, hence we could replace with bool
• Assuming UPDATE when any message was removed from the stack is a good enough approximation, hence we don't need a return value at all

Similarly, Adapt's message and timer handlers can also get away without using any return values (on the assumption that a timer is only requested when it is likely that something will happen, and that Adapt only makes things happen by adjusting its state; these appear reasonable assumptions given the intended usages).

Also, replace a couple more instances of custom widgets in examples.

Also increase the default font size to 16px (12pt).