build issues with appveyor

The working appveyor yml file is at
https://github.com/hn-88/ViewportSavercWin64/blob/master/appveyor.yml

The post-build artifact (exe file) can be downloaded from the Appveyor site due to the addition of the last "artifacts" section in the yml file.

The build issues were probably due to missing the OpenCV.props in some build configurations in
https://github.com/hn-88/ViewportSavercWin64/blob/master/ViewportSaverc.vcxproj

For example, line 72 has

<Import Project="qhy.props" />

<Import Project="OpenCV.props" />

while line 77 only has

<Import Project="qhy.props" />