implement action-based click events

[MaxVerevkin]

Closes #1519

Example:

[[block]]
block = "music"
# Click anywhere to play/pause
[[block.click]]
button = "left"
action = "play_pause"

TODO:

• Improve docs
• Mention breaking changes

[MaxVerevkin]

I thick this + #1605 can be used to register click events for specific placeholders:

format = " $icon.str(name:icon) $value "
[[block.click]]
button = "left"
widget = "icon"
cmd = "..."

Not sure how useful this is though.