Add button to copy code snippet

[ryonakano]

Fixes #5
Closes #31

Differences from the previous PR:

• Use a label instead of the edit-copy icon, so that we can also use it to tell users the text being copied
• Do not swallow Ctrl + C in case someone selects some part of the snippet and press that hotkey
  • Now we require GLib >= 2.78 because of Timeout.add_once ()

I tried the button to be similar to the one written in the Prior Art of #5, but I'm not familiar with CSS so design feedback is especially appreciated.

[zeebok]

I won't merge in case there is design feedback, but the code looks good

[ryonakano]

@danirabbit Thank you for your detailed reviews! I addressed your comments.

I don't have PCs with a touch screen so I can't test if the copy button get visible on these devices. Could you test that if you have one?

[danirabbit]

Yup that works for me! Can access the button with mouse, keyboard, and touch. Nice job :)