Updated layout in Demo for iPad-compat

• Modified the constraints in DropdownMenuDemo’s Main.storyboard to be superview-, guide-, and sibling-relative (instead of hard-coded widths/heights) so that the demo’s layout simply expands horizontally on iPads.
• Changed project settings to “Devices: Universal” (instead of iPhone-only), keeping Main.storyboard as the layout for both (as opposed to a distinct iPad storyboard).
• Changed iPad Device Orientation settings to allow all 4.