Returns existing tab index. Creates a new one if it isn't opened and returns its index
        otherwise.

def __addTab(self, filePath):
        """Returns existing tab index. Creates a new one if it isn't opened and returns its index
        otherwise."""
        for i in range(self.tabBar.count()):
            widget = self.pages.widget(i)
            if not widget.isStatic and filePath == widget.filePath:
                return i
        tab = SubtitleEditor(filePath, self._subtitleData, self)
        newIndex = self.tabBar.addTab(self._createTabName(tab.name, tab.history.isClean()))
        tab.history.cleanChanged.connect(
            lambda clean: self._cleanStateForFileChanged(filePath, clean))
        self.pages.addWidget(tab)
        return newIndex