RCP launcher: support additional VM args configuration file

[serge-rider]

Eclipse RCP apps have main configuration file eclipse.ini (or <app-name>.ini) where users can change default launch parameters.

The problem is: every time user reinstall application it loses all changes in this file.
Proposed solution: allow users to create separate default configuration file which will remain after application uninstall. Exact location must be application-specific, it maybe something like ${user.home}/default-eclipse-config.ini.

This will significantly simplify life of users who perform frequent application update or who use some automation for app install on end-user machines.

Original feature request in DBeaver project: dbeaver/dbeaver#35080
I think it will be a useful addition for Equinox infrastructure.

Proposed solution is implemented in PR #690

Currently we are trying to build custom Equinox launcher for DBeaver with these changes on our side. But it is quite tricky because of sophisticated compile/link procedure on different platforms. It would be wonderful to have it in the upstream codebase.