build: Allow devel app IDs and use consistent variables

Merged Hari Rana requested to merge improve-buildsystem into main
  • Change all occurrences of application_name to app_id
  • Reduce duplicated statements
  • Change app ID to io.gitlab.theevilskeleton.Upscaler.Devel
  • Add unsuffixed_app_id variable to have a default app ID

There are some bugs so far:

  1. Wrong icon
  2. Keyboard Shortcuts grayed out
  3. Desktop entry does not show up
Edited by Hari Rana

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