Use blueprintjs tabs instead of self-implemented ones

[ning-y]

We (or at least I) missed that blueprintjs already has a tab component. I'm mostly referring to the side-content of the workspace component.

[remo5000]

I found that the behaviour of it is a bit iffy, but we can try it out first

[alcen]

Planned - this should resolve #291 and increase functionality of the side content tabs

[alcen]

Resolved in #740!