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.

Adapt wait time for additional block updates based on that (update
happens immediately after user interaction, while there is a longer
wait time for 'Pull'-updates).

Also rename 'blockScript' to 'scriptBlock' and move it to
'QBar.Blocks.Script'.