Fix calculating the size of vertical toolbar in wxMSW.

Use the width of the largest toolbar item as the toolbar width, not the width
of the first one. The implicit assumption that all items had the same width
was wrong and resulted in items wider than the first one being truncated.

Closes #3788.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76034 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775