No Visual Studio 2017 support in Windows-MSVC.cmake, FindwxWidgets.cmake and InstallRequiredSystemLibraries.cmake

Following issues were found:

• MSVC15 was not defined in Windows-MSVC.cmake. MS C++ compiler versions:
• VS2017 - 19.10
• VS2015 - 19.00
• Add MSVC15 conditions in FindwxWidgets.cmake:

...
if(MSVC15)
  set(_WX_TOOLVER 141)
elseif(MSVC14)
...

• Add MSVC15 conditions in InstallRequiredSystemLibraries.cmake:

if(MSVC15)
  MSVCRT_FILES_FOR_VERSION(15)
endif()
...
if(MSVC15)
  MFC_FILES_FOR_VERSION(15)
endif()
...
if(MSVC15)
  OPENMP_FILES_FOR_VERSION(15 141)
endif()
...