Tools: base model with 'changed' state

[gaperton]

Model is 'changed' in case it's modified with 'set' after 'save' and 'fetch'. Flag will be cleared upon save/fetch call.

[gaperton]

Workarounds are proven to be extremely painful, unsafe, and hard to maintain. It needs to be implemented in the system core. Will make implementation soon.

[gaperton]

Will be implemented as extension, not in the core system.