Allow customizing vertical toolbar

[bilbo]

JOSM have horizontal toolbar which can be customized in preferences and vertical toolbar, which unfortunately cannot be customized. Recently I installed a plugin which (as part of its functionality) added some buttons to vertical toolbar, causing the toolbar to be larger than available space, thus requiring me to scroll it. However, the horizontal toolbar still have plenty of space, therefore it could be useful for me to move some rarely used buttons from vertical toolbar to the horizontal.

I think vertical toolbar should be customizable in preferences too.