altTab: Avoid spurious scrolling arrow in thumbnail list

Sometimes the thumbnail list can show a scrolling arrow despite there not being anything to scroll. This is due to SwitcherPopup determining whether to show these arrows by checking if the natural width of the list is bigger than the allocated width. The allocated width of each thumbnail box is fixed to 256, but the natural width can exceed this depending on the length of the title label. Fix this by setting a max width on the title label.

Fixes #1834 (closed)