Support a global preferences file in ~/.bazel/eclipse.properties for defaults for all new BEF workspaces

[plaird]

This is a postdated issue, the implementation has already been commited here: https://github.com/salesforce/bazel-eclipse/blob/master/plugin-core/src/main/java/com/salesforce/bazel/eclipse/preferences/BazelPreferenceInitializer.java#L80

As a BEF developer, I create a lot of BEF workspaces and wanted a central preferences file that would override the BEF default preference values. This global file is only used during workspace creation.

~/.bazel/eclipse.properties

Documentation is here: https://github.com/salesforce/bazel-eclipse/blob/master/docs/using_the_feature_preferences.md#global-preferences