Change to using contextmenu plugin

[Alpvax]

U̶n̶a̶b̶l̶e̶ ̶t̶o̶ ̶t̶e̶s̶t̶ ̶o̶n̶ ̶m̶y̶ ̶p̶h̶o̶n̶e̶ ̶b̶e̶f̶o̶r̶e̶ ̶c̶r̶e̶a̶t̶i̶n̶g̶ ̶P̶R̶,̶ ̶s̶o̶ ̶p̶r̶o̶b̶a̶b̶l̶y̶ ̶d̶o̶n̶'̶t̶ ̶m̶e̶r̶g̶e̶ ̶y̶e̶t̶.̶

Ready for merge.

[Alpvax]

I would recommend squashing and merging (So only the first commit message is listed in the history).

[Trymunx]

I'm happy to merge if:

• you make it possible for me to use my own class name for context menus. It might be nice if this could be scoped so different context menus could be used, although if it's defined only on the root instance of the app this wouldn't be possible anyway (and it wasn't before).
• it's definitely possible for me to overwrite the styles with my own ones. It might be nice to keep the styles set on it as default context menu styles, but these should have colours set, not commented out!

[Alpvax]

What should the default colours be set to?

[Trymunx]

I think default colours should be white and grey. Most context menus tend to do this. You could go for blue instead of grey as the highlight colour, as that's common for focus in browsers, but don't think it looks as nice.