Move the popup description when too close to edge

Description displays on the right side of the popup if it would otherwise
try to display it off-screen, offering a preliminary solution to the problem I brought up in issue #112 (but not addressing the line-wrap issue that was discussed). 

Usually the text of the hover button looks something like "desc: text". When the description is on the right side, a colon doesn't make sense. I'm not sure what the preferred way to do this would be, so I just went with "text [desc]", but that's easy enough to change if needed.