send email using the default email client instead of our own

another improvement from the geysigning project is the ability to send email using the user's regular email client. to quote their README again:

In contrast to monkeysign, xdg-email is used to pop up a pre-filled email composer windows of the mail client the user has configured to use. This greatly reduces complexity as no SMTP configuration needs to be obtained and gives the user a well known interface.

This seems like a low effort, high benefit feature that we could implement easily.