Feature: add actions to the layout

[a-kenji]

Make it possible to specify certain actions in the layout:

Start with:

• add actions to be run after a layout is constructed
• add actions to be run after a tab is created
• add actions to be run after a pane is created

Depends on: #771

[alfredopalhares]

I second this feature, it should be very useful!

[jurmc]

I'd suggest also adding actions executed on layout/tab/pane destruction.

My personal use case for this is to be able do do some cleanup on panes with remote sessions and also panes that started some containers.

[n-029894]

Actions on attach to session would be nice too.

Specifically, I'd like to specify which named tab to be selected when attaching to either a new or existing session.