window.py should not deal with per-file functionality

[Griffon26]

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Currently window.py adds and removes open files to a QFileSystemWatcher and it also handles the fileChanged signal for when an open file has been modified.

Because there is always exactly one file per tab, it makes more sense to make all of the above the responsibility of the tab instead of the window. If the window needs to know about any of this, then the tab should offer a solution (functions, signals) but the window shouldn't require knowledge of tab's implementation (no more self.currentTab.editBox.document().setModified(True))