Add a page object for the Live Data Properties advanced panel

Problem

xwiki-platform-livedata-test-pageobjects has a page object for two of the three Live Data advanced panels: FiltersPanelElement and SortPanelElement, opened by LiveDataElement.openFiltersPanel() and LiveDataElement.openSortPanel(). The Properties panel, which is how a reader displays a column back or hides one, has none.

A functional test that needs to assert what the panel offers, or to display a property back, therefore has to reach for getDriver() and raw By lookups, which the testing strategy forbids in a test class.

Proposal

Add PropertiesPanelElement extending AbstractLiveDataAdvancedPanelElement beside the other two, with LiveDataElement.openPropertiesPanel() to open it, exposing the panel's actions: list the properties it offers, tell whether one is displayed, and display or hide one.