Improvments to the Share dialog.

Full Screen is not shown as an option for no-owner files, since
the URLs won't work.

Hyperlinks with tooltips are added explaining the three types of
sharing, and letting you go to them right away.

As secondary feedback when you click a Copy button, we also
select the URL text.
do not shorten if the file is too long
Merge pull request #42 from weihang7/master

Do not attempt to shorten if the file is too long.
Slightly longer tooltips to explain the three types of sharing.
Scroll long URLs to the right in the Share dialog.
tooltip consistency
Merge pull request #41 from PencilCode/sharefixes

Improvments to the Share dialog.