poll screen: don't show all

The widet show different widgets depending on state and configs
Select screen: show vertical scrollbar if needed
Settings: apply "remember last vote" inmediatly

If a poll is running, reload the screen when the setting change,
is the only way to uncheck all the radio buttons.
The callback methods for the radio buttons and the vote button
are moved to the PollCanvas class, to improve the encapsulation.
poll screen: simplify margins
chart: center pie