Add ability to approve a MR

Problem to solve

Add ability to approve a MR.

Proposal

For me, the most intuitive way for doing this is to add the option when right-clicking on the MR name, same as the option "Checkout MR branch"

Further details

In some teams, a MR needs the approval of some members. This would ease this process.

Links / references

I saw there was a proposal in #227 (closed), but was closed as duplicate of #63 (closed). Unfortunately #63 (closed) did not add this feature.