Support open dialogs that aren't modal to a particular window

[arifd]

Currently, calling druid::commands::SHOW_OPEN_PANEL with a Target that isn't a window, will result in nothing happening, and no warning is produced.

A PR already exists to add a warning, but as per @cmyr, here, global open dialogs should be a thing.

[cmyr]

Some additional notes on this:

Part of why this is complicated is that there's some platform behaviour to consider. On macOS, it is normal to have an application open with no open windows; and even with open windows there is a distinction between a modal open dialog (that is modal to a window, for documents that will be opened in that window) and open dialogs that are scoped to the application level; these are generally used when the result of opening the file will result in a new window.

So on macOS, it should be easy enough to make this change.

I'm less sure about other platforms? In practice this may not really matter. We could just add a flag in the API that indicates whether a given dialog is scoped to the window or the application, and if this isn't meaningful on a given platform we can ignore it.

In any case, before really trying to implement this I would want to have a better understanding of the behaviour of the non-mac platforms, so that we can figure out a design that makes sense.