Add editing capabilities to config-ui application.

Configuration entries can be added and removed, and their values
as well as their names can be changed. The latter feature
means the GUI interface is now actually more powerful than
the command-line version.

Change-Id: Ic95954fe7bcae727448d25cfba833792ba47b5cb
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
introduce Transformer::product()

Simplifies the calling code.
Remove pointless nullpointer check from jscommandexecutor.

Change-Id: I867181d2b750f32f04376ce860f5dee6555d3e33
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>