Option to lock custom properties to just the predefined ones

[bjorn]

The ability to add any custom property to any object can be nice, but it can also lead to inconsistencies and possibly bugs, for example when typos are made or a wrong type is chosen. These problems could be avoided by an option to only allow the use of predefined custom properties.

Suggested as part of #129.

[eishiya]

This would be especially useful to me in combination with being able to set default properties on layers (#1410), as I've somehow ended up with a bunch of maps that have string properties when they should be ints ._.