Clean up multiple installs on Windows

[martyngigg]

This issue was originally TRAC 10060

There are some issues where that could make this process cleaner:

• the uninstaller just assumes that the install is in C:\MantidInstall and removes this one, regardless of where it actually is;
• environment variables are set to the last package that is installed - particularly PATH has the bin directory so that both versions will try to use the same libraries.

Solving these problems would mean that having multiple versions is simply about installing once and then installing to a different directory.

We could possibly avoid both of these issues by indirecting the exe calls through scripts, much like we do with RHEL6. This would allow us to set environment variables for a single process rather than globally for the system. We need to investigate things like hiding the console window that would be produced and also how this impacts on debugging for developers.

[martyngigg]

This has been fixed in #13337.