build: Allow devel app IDs and use consistent variables

• 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