Add copy & paste to context menu and enable context menu in per file view

[g-maxime]

Fixes #133

[dericed]

Can you add keyboard shortcuts to this? In my first try I was trying Command-C and Control-V but it didn't work, but the right click context menu's "Copy/Paste" did.

[dericed]

Hi @g-maxime, this is better, but the interface doesn't seem to update after I paste.
Currently, I can select a field, Command-C, select a field of the same column but different file, Command-V. Then there is no update in the GUI; however if I select another field or interact with the gui somehow then the pasted value appears, so there's a refresh missing directly after the paste. If I use "Paste" from the right-click menu, then the pasted value appears right away.

[dericed]

LGTM