Fix #9 avoid clipping Runeword popup at bottom of viewport

[fabd]

• check the popup height against the viewport height
• move the popup vertically to fit in the viewport
• if the popup is really tall and user has increased zoom settings, it's possible it is too tall to fit, in this case stick it to the top and let it clip at the bottom of viewport
• also use a little gap, looks nicer than having the popup stick exactly on the edge of the viewport