improve copy&paste

[pkra]

Ideas from the backlog.

• share mechanisms (webintents (dead), mathproc://)
• microdata support (rdfa, schema.org)
• investigate copy&paste with web components / custom elements etc.
• copy&paste via Clipboard API, https://github.com/zenorocha/clipboard.js etc -- cf. [Menu] "Copy Math As" #1663

[pkra]

Here's a recent article about the new Selection API http://updates.html5rocks.com/2015/04/cut-and-copy-commands IE10+ and Chrome support it now. (This was mentioned first by @Yaffle on #442, so thanks! made me keep an eye on it)