Use portal to request access to camera

### What Happened?

Instead of using `--device=all` in the Flatpak manifest, we should ask the user for permission to access the camera and microphone using [`org.freedesktop.impl.portal.Access`](https://docs.flatpak.org/en/latest/portal-api-reference.html#gdbus-org.freedesktop.impl.portal.Access)

### Steps to Reproduce

1. Start Camera app
2. It instantly activates the camera - or fails silently in case there is no camera available

### Expected Behavior

1. Start Camera app
2. The app should check if it has the required permissions and the needed hardware is available
3. If permissions are missing, we should ask the user to allow access to the hardware
4. If anything goes wrong, we should display a meaningful message / placeholder so the user knows why the app is not capable to do its job

### OS Version

7.x (Early Access)

### Software Version

Latest release (I have run all updates)

### Log Output

_No response_

### Hardware Info

_No response_