Automatically update MSBuild platforms folder

[phofman]

It's quite messy, to ask developer to manually update the related MSBuild folders with new platforms etc. each time any new change to the plugin is published. There should be a way to make this process automated, so they don't bother with it anymore.

Current proposal:

• embed the tools and platforms dedicated to Visual Studio edition inside the VSIX
• extract it, each time version inconsistency (between plugin and build-tasks) is detected (instead of just creating an error message, as it is now).

Expected problems:

• admin rights are required to play with ProgramFiles folder and VS doesn't have them and it should be possible to update MSBuild without running VS as administrator

[phofman]

OK. I created an utility (in plain C), that asks for admin privileges and performs copying of MSBuild platform files for respective Visual Studio version. This utility is digitally signed to look better in UAC using my Signature project.

Things still missing: a) updated documentation, that no manual actions are needed b) verify that command-line build scripts are not broken c) error message that 'MSBuild needs update' should disappear immediately after an update; currently it will go away, when new project is opened or created after d) make the MSBuild platform-installer-utility open-sourced too (?)

[phofman]

Implemented on develop, tested on Win8.1 and published via VS Gallery.