The plugin should provide an input field for setting arbitrary additional
system properties, just like for example the ant build does.

Currently it is not possible to set system properties except for
grails.work.dir, project.work.dir and server.port. There are many other
interesting properties that users might need, including user-defined
properties.

Specifying -D options as target options does not work either: Using "-Dprop=foo
war" as target (with the quotes) generates an error about missing target, and
"war -Dprop=foo" treats the -D option as a target option and has no effect.

With the general feature, the three properties mentioned above would no longer
require separate input fields (which are left blank for most builds anyway).
This is clearly a more scalable solution.