Fixed dialog units <-> pixels conversion.

Fixed dialog units <-> pixels conversion.

The computation is now identical to Windows native one. To achieve this,
MSDN-recommended GetTextExtent() call is used instead of GetCharWidth().
wxMulDivInt32() is used instead of integer arithmetics to achieve
correct rounding.

Use toplevel parent's font instead of window's own. This makes more
sense, as dialog units are defined for TLWs, not individual subcontrols.

Also fixed wxMSW's wxButton::GetDefaultSize() to compute dialog units
correctly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63251 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775