Ability to sync multiple branches at one time (BB-10663)

When working with a lot of branches (e.g. customer branches), syncing becomes quite time-consuming since you have to click Edit>Sync for each of those branches and for each branch you have to wait half a minute to complete.

It would be great to have an option for syncing multiple branches with "one click".

E.g. select multiple branches with a checkbox and then Edit>Sync