Implement functionality to upload files

[benediktscheffel]

The user should be able to upload pdf files to the apps collections.

Current situation: Right now there is no functionality to upload files.

Target situation:

• Add entry in a collections context menu to upload files
• Add functionality to upload a file from the devices storage. There should be flutter libraries to handle files from devices storage

[benediktscheffel]

The option to add Files to a collection is implemented. In the Context Menu of a collectoin the user can choose 'Add File':

The platforms file picker opens on click on this button and the selected file is uploaded to the chosen collection afterwards: