Fix wxMSW wxSlider best size calculation and label layout.

Fix wxMSW wxSlider best size calculation and label layout.

The best size calculation was wrong as the min/max labels height was
unnecessarily added to the slier total height even if though these labels are
positioned alongside the slider itself in horizontal layout case.

The slider also managed to position its value label completely outside the
space allocated to it (and the bug in DoGetBestSize() might have been an
attempt to work around this), don't do this any more. This also fixes the
wrongly centered vertical position of the min/max labels.

Closes #13291.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68207 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775