ve.ui.PanelLayout: Set default to span accross available space

Without these properties set, the layout is collapsed into a 0x0px square due to the
parent having overflow-x: auto; and no width/height set.

Change-Id: I77aef8afa3327b11f453196badcdfad3f51f7c3d
Merge "ve.ui.PanelLayout: Set default to span accross available space"