ImageStack: add an option to search for an url

[payno]

It would be nice to have a search bar associated to the UrlList.

Something like:

    def __init__(self):
        ...

        self._searchWidget = qt.QLineEdit(parent=self)
        self._searchWidget.setPlaceholderText("search")
        search_icon = icons.getQIcon("search")
        self._searchWidget.addAction(search_icon, qt.QLineEdit.LeadingPosition)
        ...
        self._searchWidget.textChanged.connect(self._filterUrlList)
       ...

    def _filterUrlList(self, *args, **kwargs):
        """Filter the url list according to the 'searchWidget'"""
        filter_str = self._searchWidget.text()
        filter_str = filter_str.lstrip(" ").rstrip(" ")
        for item_index in range(self._urlsTable.count()):
            item = url_list.item(item_index)
            item.setHidden(filter_str not in item.text())