Optimize elements list filtering to retrieve an element label only once

[LeonarddeR]

As discussed in this thread on nvda-devel:

What do you think about the following though, related to the elements list? IN browseMode.ElementsListDialog.filter, label is called twice in every for element in self._elements loop, namely:

• if filterText and filterText not in element.item.label.lower():
• item = self.tree.AppendItem(parent or self.treeRoot, element.item.label)

I'd like to suggest create a local label variable which contains element.item.label, and than do the returning and filtering with that variable. This will, if I understand it correctly, call element.item.label only once instead of twice, which can improve performance on complex label generation as in virtualBuffers.VirtualBufferQuickNavItem.label.