Make tabs toolbar no longer have a context menu

Summary:
Adds a contextMenuPolicy of Qt::PreventContextMenu to prevent the
tabs toolbar from showing a context menu that allows it to be
hidden.

Backport of Core PR11169
Completes T443

Test Plan: ninja

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1966