File upload improvements

[henrik429]

• [ ] Iterate through tree more efficient (e.g. along a path instead of the current brute force approach) for functions addFileToTree and removeFileFromTree
• [ ] Save all expanded nodes such that after adding/removing files (and therefore, after collapsing) they are expanded again all at once
• [ ] Find a way to allow filenames with white spaces (instead of replacing them with underscores)
• [ ] Allow upload of files with sizes >100MB
• [ ] Use different icons for different file extensions

[MoritzWeber0]

• [ ] Adjust margins between folders and files (they are different)
• [ ] Set minimum width of upload files windows, so that it doesn't get wider when expanding folders
• [ ] Mark directories without children as not expandable