Python editor should detect externally modified scripts

[RenWal]

Describe the bug
The Python editor currently does not monitor the file on disk, meaning if you modify the file in another editor (or you have it in, say, a git repository and pull in some commits) and then click the save button in HAL, the external changes are lost.

Most IDEs can do this, so this will probably be unexpected for most users.

To Reproduce

1. Create a Python script in HAL and save it.
2. Open the script in an editor of your choice and change something.
3. Hit save in HAL.
4. Open the script in the external editor again and see that your changes have been overwritten.

Expected behavior
The editor must not overwrite a file that has been modified externally without asking for confirmation. The editor should display a subtle notification (no focus-stealing dialogs here) asking whether the user would like to re-load the file whenever HAL senses an external modification.

Additional context
gedit does this well, maybe take a look there.

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[dertob]

Detection of externally modified, moved or deleted scripts is working in branch 'feature/python-editor-file-modified'. The user is presented with the choice to reload a file from disk or ignore the external changes.

The only thing missing in this feature is that tabs need to be marked as modified as soon as

1. the modification on disk gets ignored by the user.
2. the script has been moved on disk.
3. the script has been deleted on disk.

Due to #157 tabs cant be marked as modified with 100% assurance though. I suggest fixing #157 first instead of merging this right now.

Do not merge. Needs some adjustments after graph view integration and changes on the global file status manager.