Factor out text measurement from wxDC and wxWindow into wxTextMeasure.

Factor out text measurement from wxDC and wxWindow into wxTextMeasure.

Add a new private wxTextMeasure class implementing methods for measuring text
and move the often duplicated (but not always identically) code for doing the
same from wxDC and wxWindow into it.

Currently this class is only really implemented in wxMSW and wxGTK.

Also extend the test for text measuring functions and rename it to
MeasuringTextTestCase from MeasuringContextTestCase as it's not wxGC-specific
any more.

Closes #14705.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72699 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775