make modals closable by clicking the background

[AraxTheCoder]

This adds the ability to close a modal by clicking the background. Also setState got replaced with ValueListenableBuilder so that the child does not need to get rebuild every time the offset changes.

Closes issue #498

Testing instructions:

1. Go to the tasks app home page
2. Click on the navigationbar option 'Neu'
3. Click on either 'Task' or 'Patient'
4. Click on the semi-transparent background

Additional notes:

• replaced Scaffold with Material widget since the AppBar of the Scaffold was not used and so the widgets have the same effect with the exception, that Material does not take up the whole screen.

[github-actions[bot]]

BrowserStack

Commit: 58ac2ff

helpwave tasks

• Android (13.0)
  • Google Pixel 7 Pro
• iOS (16.0)
  • iPhone 14 Pro

    helpwave impulse

• Android (13.0)
  • Google Pixel 7 Pro
• iOS (16.0)
  • iPhone 14 Pro