Combine drag and drop and select file button into one.

[GerwinBosch]

The current implementation looks nice, however, they serve the same function and cause some confusion. (At least, I tried to click the drag and drop surface, expecting it to open the dialog)

Suggested implementation:

• Increase the size of the drag and drop box (I'm lazy :P)
• Make it so that clicking the drag and drop box also opens the file selection
• Move the button (or its text) within the drag and drop box
  • To keep the quality of the rest of the components, we can probably use a Mui buttonbase for the whole drag and drop box.