Several changes for the latest Curse and WoW versions

[timwienk]

I've just pushed my local changes to Github, in the hope that it may help others. I haven't split them up in separate branches/pulls, so feel free to pick & match if you're interested. I can also do some splitting up if that would help.

Probably good to note: I've only run this on Python 3.7 (and not with pipenv). I would imagine it works fine with pipenv on Python 3.6, though.

Summary of changes:

• Added keyboard shortcuts (Del for "Remove from list", Shift+Del for "Remove addon", Backspace to clear a cell)
• Small import QOL improvements (Cyan background for newly imported addons, always update Interface version from TOC files)
• Sortable columns [#13]
• Fixed some TOC parsing errors (problems with BOMs and Curse Project IDs)
• Changes to work with latest curseforge.com (HTTPS URLs, HTML structure changes) [#75]
• Very basic and naive support for WoW Classic and checking whether an addon is for Classic or not (but it works) [#78, #79]

[Ironlenny]

I can confirm it works for Python 3.6.8 and pipenv.