Application manager: Do not assume Qt will be always present

[jaimergp]

We currently use Qt for the UI because we know we are going to ship it as part of napari. However that's a big dependency that not all projects might want to carry. We should strive to provide a barebones agnostic manager that can operate with other GUI toolkits, or without any at all. Maybe some TUI like Rich is enough.

[dalthviz]

Maybe using app-model to define the application capabilities/commands/actions available could be worthy here @jaimergp ?

[jaimergp]

Good idea!