GoClipse should use GOROOT environment variable

[rillig]

• Windows 10
• Eclipse Oxygen M3
• GoClipse 0.16.1.v201607061938

After installing GoClipse, it complained that GOROOT were not defined, even though I have defined it in the System Environment Variables.

GoClipse should take the GOROOT environment variable as default. If that is not set, it should look in the usual folder (C:\Program Files (x86)\Go* and choose the installation with the highest version number as default.

Doing this makes the installation less painful, since when GOROOT is not set in the Eclipse Preferences, GoClipse complains loudly when I open the Project Properties and want to leave the Go Compiler properties page, telling me that GOROOT is unset.