Consolidate app management code

[MorrisJobke]

Currently the code for managing app (install, update, remove, upgradability, fetching info, ...) is distributed across 3 classes:

• OC_App
• OC\App\AppManager
• OC\Installer

We should consolidate this into a single class to not spread code and functionality as well as reduce the risk to re-implement existing code, because it is no obvious where to find which code.

[MorrisJobke]

cc @nickvergessen @rullzer @blizzz FYI ;)

[nickvergessen]

Sounds good 👍

[MorrisJobke]

One little step is already done in #5833

[MorrisJobke]

From #8506 (comment)

Learned some funny things about our terminology the hard way:

• isInstalled, getInstalledApps, isEnabledForUser
• first and last one check if the app is enabled
• the other one checks if the app code is present in any of the apps folders 🙈

I added a little PHPDoc comment and we should look into unify this stuff.

[MorrisJobke]

See also #9324