LVGL Flow: Add event handling for pages/screens

[fietser28]

LVGL has several events specific to screens (load/unload). These events can currently not be used in flow because there is no event handling and interface for it.

It is convenient to have this interface available to trigger actions/flows when a screen is loaded (displayed) or when left.

In the UI this probably means:

• the properties tab of a screen get an events section similar to other widgets.
• If flow events are defined in the properties handles appear at the screen borders, similar to other widgets.

This issue might also prepare for partial implementation of #277?