suggestion: context object for layerlist and viewer to be moved to app model

[kne42]

currently, the layerlist and the viewer have a _ctx ... but I think it might be better suited on the app itself, which will listen for change events of those objects to update it's context?

[kne42]

welp i just accidentally clicked somewhere on the page and it made this sooooo

[liu-ziyang]

For context, this was created from the TODO list for app menu migration. Supposedly it should be tracked as a draft issue to help planning but was mistakenly created. If not too misleading or distracting, I will leave this open and get to this while we march on the app model migration