IDE improvements

[kkoomen]

• Fix bug when a C and Python file are both active in split view and then the user refreshes, which ran into an error.
• Delete files recursively when deleting a folder, including closing it's tab if open.
• Small styling improvements:
  • use dotted border instead of solid for file-tree inline-edit
  • use bold font for danger button in modals
  • add spinner to the run-button to let the user know it is loading up a worker