Tag assets in Asset Manager to prevent unloading

[sooooosh]

Some units have other assets as dependencies, that aren't actually placed in the level and seen as "unused" by the Assets Manager. So when using the "Remove and unload unused assets" function, it removes those assets even though they might be needed for other units to work properly.
Being able to manually tag assets so that they are ignored by the unloading would greatly help.
A button could be added in the Selection Actions called "Mark as used", which will then remove the unused warning and the yellow background. Of course it should be reversible to unused again.
In the add.xml it could be handled with a parameter, like used="true" , important="true" , or something similar.