More options for 'Class of' property in Custom Types Editor

[ChristianPervoelz]

Is your feature request related to a problem? Please describe.
It's not really a problem, but...

My (Tiled) project currently has about 100 custom classes for layers only, where half of them is for tile layers and the other half for object layers. All these classes have 'Class of' set to 'Layer'.

Now, I add a new layer (tile or object) and set the class for it. The drop down list is (of course) really, really long, but half of the items in that list is not applicable, as they are not dedicated to be used for the specific layer type.

Describe the solution you'd like
It would be great to replace the 'Layer' item in the 'Class of' selector of the custom type editor by four different values (one for each layer type).

Describe alternatives you've considered
My current approach is to use a prefix for the layers (e.g. PropertiesLayer_A, TileLayer_A, PropertiesLayer_B, TileLayer_B, ...)