[LVGL] Make hidden widgets partially visible in the editor

[zalexzperez]

On interactive screens, we may have many widgets hidden by default with flow lines coming out of them.
Not being able to see them at all can be inconvenient.

This is the default state for one of my screens:

With the requested feature, it would look roughly like this (disregard the transparency on the default state content):

In the Settings, there could be an option called "Hidden widget visibility", allowing users to adjust the transparency as a percentage.