Link hover doesn't show link tooltip

## Problem

Hyperlinks in emails don't show a tooltip when hovered over, which seems like a security/safety issue if you can't see exactly where a hyperlink is leading to.

## Proposal

When mouse hovers over a link, show a tooltip with the full URL.

## Prior Art

If I remember correctly the old Geary-based client did this, but I'm not able to get a screenshot of it now. The ideal implementation of this would be to use the system-wide black tooltip and just show the whole link beneath the mouse on hover, but if it needs to be more in line with other default app behaviors, it should appear on the bottom right, like the file explorer.