invalidate cached client size when scrollbar visibility changes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78113 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
avoid multiple emissions of the GtkAdjustment "changed" signal when configuring scrollbar

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78114 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775