Enhancement #9 Export Current Block To a File

[zeltrex7]

#9 Added the button at the top right corner to download content of block to a file

[heyman]

Hi!

I don't want to merge the UI changes (e.g. download buttons, fontawesome dependency). I'd like the feature to be implemented as an exportCurrentBlock command that can be invoked through Menu -> Block -> Export Current Block.

There should probably be a call to revokeObjectURL somewhere.

This PR also includes unrelated changes (e.g. electron-builder version change, indentation changes).

[zeltrex7]

@heyman will try to build that way.