Use more exact paths for ivysettings.xml and build.properties (#13616)

Defining the paths to be relative to the workspace is needed for
compatibility with older versions of IvyDE / Eclipse. All other
dependencies are already defined in this way, it's only this new gwt
dependency that isn't consistent with the others.

Change-Id: Iad976566fc7927745fe5534951be1f476c810ff5
Tweak quick setup instructions (#13616)

- Change the order to install IvyDE before importing the project
- Explicitly mention the Ant tasks since it's such a common mistake

Change-Id: I1a9edc39f72c5fd4b31cdfe3db9b4fa87d6d8cf2