[feature] hide windows that are in the scratchpads

[sforli]

I realized it would be great to have an option for wlr/taskbar to hide applications that are in scratchpads. For that it would be sufficient to have an option like this in the configuration:

"ignore-scratchpad": true

It feels very complementary to the waybar-sway-scratchpad module, for example, which handles windows that are in the scratchpad, while the taskbar shows only those that are not.