Fix wxComboCtrlBase::DoGetSizeFromTextSize() performance regression.

r72935 introduced code that created a temporary wxComboBox control to
take measurements. This is very expensive in MSW and adds noticeable
delay when creating more than a few controls.

Replace with equivalent wxMSW code that computes the height in the same
way other wxMSW controls do.

The wxGTK version cannot be eliminated in the same way, so at least add
some basic caching there. It's much less of a problem, because GTK+
controls creation is much cheaper than MSW one.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74658 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775