Make file-adding experience consistent across our tools

[olizilla]

All three apps provide drag'n'drop and and a file picker dialog to add a file, but there are problems with each one. What is the ideal UI/UX for adding files to your IPFS repo.

This is closely related to the next step Share Files via IPFS discussed in #34

The current state of things is described here: https://github.com/ipfs-shipyard/pm-ipfs-gui/blob/master/research/README.md#add-files