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'.

Use tag parser for 'blockScript', 'persistendBlockScript' and 'qbar pipe'.
'blockScript' no longer handles (and allows) exit codes other than 0.

This reverses the control flow between 'Cli' and 'Server'

This removes Pango support for script blocks. Persistent scripts don't
have an alternative way to format their output for now.