Implement Paste UI for Selecting Recipient Peer

[schlagmichdoch]

This pull request enhances the paste clipboard functionality introduced with #108 .

Main changes

1. adds text support to clipboard _onPaste() functionality and fixes issue where pasting into message dialog modal sends item directly
2. adds clipboard ui / paste mode: When something is pasted user can simply select the recipient to send item

Overview

Overall this pull request extends the possiblities to share items with peers via snapdrop without conflicting with the standard ux at all. As @sonicblis suggested, a paste UI is added to let user select the recipient directly.

Workflow

1. copy file or text you want to share
2. open snapdrop.net
3. paste elements (e.g. via CMD+V)
4. select recipient

This PR supersedes #488