"Cancel" button shouldn't close Eclipse but go back to workspace chooser

If the selected workspace is invalid in terms of
IDEApplication.checkValidWorkspace(), let user select valid one and
trigger re-launch of Eclipse with valid workspace location.

Doing so:
- made Workbench.setRestartArguments() public for reuse
- removed duplicated buildCommandLine() code from OpenWorkspaceAction
- removed IDEApplication.EXIT_RELAUNCH (same as parent provides)
- removed IDEApplication.PROP_EXIT_CODE (same as Workspace provides)
- fixed "Older Workspace Version" version dialog not shown in front.

Change-Id: I6d855c1f205116f255889126bdc89b0ccb050af6
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>