Delete screenshots, feature graphics and APKs in background

Merged username-removed-124398 requested to merge NicoAlt/repomaker:delete-in-background into master

Based on !156 (merged).

Before a separate page opened which wiped all edits of the app that were not saved before.

Deleting screenshots fails because of an bug in the screenshot delete view fixed in !154 (merged).

Related to #88 (closed).

Edited by username-removed-124398