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.
I consider the following scenario and I find it better to check if a file exists against the path captured at the method call rather than at the process start:
1. There is no `sh.exe` installed and the user gets a red label on settings page.
2. The user installs 'sh.exe' and clicks on the red label to verify if 'sh.exe' can be found.