[FEATURE] Option for fallback icon pack

[charliefrance]

Describe the feature

Add the option for a fallback icon pack for when you have both homescreen and app drawer set to themed icons and you don't want system icons for un-themed icons.

Why would it be useful to add?

This is possible in lawnchair 12, and also if you have it so that either the homescreen or app drawer are themed, but not both. It would be useful to people who want themed icons all around, but don't want to fall back to system icons when the icon pack doesn't support an app.

Did the feature exist in Lawnchair v2? (Play Store version)

No

Example(s)

For example, I have Lawnicons installed, but I have about 10 apps that are not yet included in that icon pack. In this case, I would for the un-themed apps to fall back to my Arcticons, which is the other icon pack installed on my device.

This could be achieved by simply allowing this menu to appear when both homescreen and app drawer is themed.

This is the menu when both are themed

Additional context

No response