[FIX] sap.ui.unified.ShellHeadItem: Prevent navigation on click

IE always interprets a click on an anker as navigation and thus triggers
the beforeunload-event on the window. Since a ShellHeadItem never has a
valid href-attribute, the default behavior should never be triggered
when clicking on an ShellHeadItem or ShellHeadUserItem.

BCP: 1482015720
CR-Id: 012003146900009254302014
Change-Id: Idb1d5eca26bae5944612666ac8a8678ce3da69c1
Merge "[FIX] sap.ui.unified.ShellHeadItem: Prevent navigation on click" into rel-1.26