Add "app update" command to modify entrypoint/documentroot/...

[martin-helmich]

• Add "app update" command to change entrypoint+docroot
• Update README

[niristius]

some testing was done although due rather current versions of everything in the studio, the amount of dependencies that need to be upgraded are very limited. I could only force PHP version to be low enough to require an upgrade during the app upgrade process.

[niristius]

For clarification - I just realized this.
I kinda hijacked this branch.
@martin-helmich initially created this branch and added the app update command to change configurations of an app, entrypoints to be precise.
What I added is the app upgrade command to trigger version upgrades for apps.
This is ready to be merged.

[niristius]

@martin-helmich I cant request a review from you since I hijacked this branch from you. So here's my written request 😄