The full date is very long compared to other blocks, which is
problematic when using qbar on devices with small or vertical screens
(e.g. on the Pinephone).

Renames PullBlock to PollBlock and moves it to the SignalBlock in
QBar.BlockHelper.
The script block is now in linewise mode by default and can be
configured into polling mode with '--poll'.

- PushBlock becomes Block.
- Block and PushBlock differentiate in their Proxy upstream type.
- Exit codes are unified as ExitBlock.