Specify Git and PuTTY locations with environment variables

In addition to Git Extensions' existing auto-detection logic it would be nice if the locations of Git and PuTTY could be specified using environment variables.

This is espescially useful in combination with a dependency-injecting package manager such as [Zero Install](http://0install.de), which uses environment variables to tell applications where they can find their dependencies.
But in this case, the "Git bash" toolbar button will not work until Git Extensions is restarted.
So the settings page says sh.exe was found but the corresponding button shows a not found error.