Disable / enable logics

In some cases it makes sense to temporarily disable a given logic and enable later again. At least I would like to disable some logics for some days but enable it again later. And I don't want to comment out the logic and restart SmartHome.py.

This patch contains the following enhancements:
* enable and disable logics with `logic.enable()` and `logic.disable()`
* check enabled status with `logic.enabled` property
* enable and disable logic using `el` and `dl` CLI commands
* show if logic is disabled when using `ll` CLI command
* add posibility to enable and disable logics via visu plugin (using `{ "cmd" : "logic", "name" : "<name>", "enabled" : 0/1 }` - the `val` attribute can also be specified to additionally trigger the logic, as this is the current behaviour)