[GUI] refactoring of config_brick_* methods

[Daniele Lacamera]

in gui.py:

config_brick_confirm
config_brick_prepare
(config_brick_cancel)

are way too long and must be splitted into smaller methods.
Consider using separate objects to abstact the configuration process.

[Daniele Lacamera (o-root-danielinux-net)]

r371 contains some rework of the prepare/confirm methods.

The code is more readable now, closing this.