Make it possible to use a relative path to GWT installation

Currently it is only possible to create an absolute path to the GWT installation.
This is causing problems because some of our developers have macs, and others have windows pc's.

When selecting the GWT installation in the plugin, it will modify the .classpath files with absolute paths like c:\.... Offcourse the mac pc's don't have a clue what c:\ is.

Without the plugin we solved this by creating an Eclipse project structure as followed

Project
|-imp
--|-src
--|-war
|-vendorlibraries
--|-gwt-codeserver
--|-gwt-dev.jar
--|-gwt-user.jar
--|-validation-api-1.0.0.GA-sources.jar
--|-validation-api-1.0.0.GA

When you then add those gwt jars to your classpath, they are relative in your .classpath file.

I actuallly managed to fix this problem, by editing the .classpath file by hand.
I replaced the lines as followed created by the plugin (multiple lines exist, this is just 1 example) :

with :

Note the / before vendorlibraries. This is the way Eclipse needs this to be (all your relative jars start with a /)

Now all is working, but Eclipse now gives the following error message :
- The GWT SDK JAR gwt-servlet.jar is missing in the WEB-INF/lib directory

This is because the plugin can't find the gwt-servlet.jar. I think because it is looking in c::\vendorlibraries instead of in the relative vendorlibraries directory.

Actually, this is not a problem for us because we don't use gwt-servlet.jar because we are talking with a C#.NET backend and are not using any servlets.

Start with the GWT installation selection in the Eclipse plugin and inspect how it writes .classpath entries and locates gwt-servlet.jar. Reproduce the setup with the relative /vendorlibraries/gwt-user.jar path, then verify that projects can use the relative paths across Windows and macOS without incorrect missing-JAR warnings.

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