New control: loading indicator/dialog for when Storage is downloading items

[WolfgangSenff]

A loading indicator should be displayed in a dialog when Storage is downloading a file. You should be able to load a LoadingIndicator node into the scene tree, give it a path to the file or files you want downloaded, and it should remain open until all files are downloaded. See if this can be done using several requests simultaneously. If not, that's okay, just make sure it's a dialog that pauses the tree but not itself nor the download tasks, and is exclusive so they can't click away from it.

[WolfgangSenff]

Should consider having a whole screen for this, in addition to a loading indicator control.