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.

Eclipse plugin の GWT インストール選択から始め、.classpath エントリの書き込み方法と gwt-servlet.jar の検索方法を調べます。相対パス /vendorlibraries/gwt-user.jar を使って設定を再現し、その後、プロジェクトが Windows と macOS で相対パスを使用でき、JAR が見つからないという誤った警告が出ないことを確認します。

索引モデルが issue の本文から書いたものです。