Rename $label to $labelElement in MWParameterPage to avoid naming conflict

Causing the label of each parameter to be missing ($label actually gets sent to the outline item
widget, for some reason). Looks like it was caused by I1abecd8a.

Bug: 70406
Change-Id: I598ffe5066d0ae8267324d4125d05514449064d5
(cherry picked from commit 160d99d9f450fd588de115f3b3304c2435f99ddb)