Fix regression with wxAuiToolBar gripper and overflow members initialization.

Fix regression with wxAuiToolBar gripper and overflow members initialization.

The changes of r72785 moved m_gripperVisible and m_overflowVisible
initialization to Init() but this was too early as these fields depend on the
window style only set when it's really created, so re-initialize them to their
proper values in Create().

Closes #14794.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72914 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775