Vue: restructure app setup

[distantnative]

In anticipation of Vue 3:

• remove importing Vue globally as much as possible
• register e.g. component instead via Vue plugin that receives app instance in Vue 3 (in Vue 2.7 still global Vue)

[bastianallgeier]

@distantnative could you have another look at the index.js and see if I fixed the conflict correctly?

[bastianallgeier]

Shit. Looks like i made a mistake, but I'm not sure what's the correct way to do it.

[distantnative]

@bastianallgeier fixed it