Entity properties aren't implemented correctly

[Julian Kalmbach]

Describe the bug
Whe you define properties, the body of the property definition is empty.

Example:

properties {
    bool("test_bool_property") {
        default(false)
    }
}

results in:

"properties": {
  "monstera:test_bool_property": {},
}

Version
Monstera: 0.4.0-SNAPSHOT16