Drop support for Windows 8.1 and WP 8.1 and use VS2017 platform toolkit

These are really old platforms. Microsoft stopped supporting WP8.1 already, user base on Windows 8.1 is very small and shrinking. I do not think that anyone will start a new project targeting Windows 8.1 or even WP 8.1. Windows 10 is the way to go for all new apps. Let's move on.

Having to maintain these platforms makes adding new features really difficult. Not only would we have to install VS2015 (a whooping 26GB install with all required packages) just to check that they build, but theoretically, we would also have to test new features on Windows 8.1 and even WP 8.1, which is just not feasible anymore.

So I propose to drop support for these platforms for future development. A support branch can be created, so those who still do care about these platforms (anyone there?) can still build it. Project files for Windows 10 version should be updated to use the VS2017 platform toolkit.

Please let me know what you think.

Review the Windows project files and current build configuration for references to Windows 8.1, WP 8.1, and the VS2015 platform toolkit. Update the Windows 10 projects to use the VS2017 toolkit, then verify that the supported projects still build and that legacy platform support is removed as proposed.