MelCustomDataUtil.py

bl_info = {
    "name": "Melsov EZ Custom Data Editor",
    "author": "Melsov",
    "version": (1, 0),
    "blender": (2, 80, 0),
    "location": "View3D > Sidebar > MelCustomDataUtil",
    "description": "Convenient buttons for addding custom properties",
    "warning": "",
    "wiki_url": "",
    "category": "3D View"}

# This add-on provides convenient buttons for adding and removing
# custom properties on objects. Facilitates multi-select adding/removing.

# REMINDER: you have to check 'custom properties' when exporting the FBX from blender
#  to include custom properties in the exported FBX.
#   Or if using the EdyJ Unity to FBX exporter, 
# https://github.com/EdyJ/blender-to-unity-fbx-exporter
# modify the code
#    in that add-on. Find where it calls the default exporter
#    add the parameter: use_custom_props=True 
#    HERE's the line in blender-to-unity-fbx-exporter.py
# Export FBX file
# params = dict(filepath=filepath, apply_scale_options='FBX_SCALE_UNITS', object_types={'EMPTY', 'MESH', 'ARMATURE'}, use_active_collection=active_collection, use_selection=selected_objects, use_armature_deform_only=deform_bones, add_leaf_bones=leaf_bones, use_custom_props=True)


import bpy

def allSelectedHaveKey(key, context):
    for ob in context.selected_objects:
        if key not in ob:
            return False
    return True
        
# Add key : values as needed  
default_data = {
    "mel-collider" : 44,
    "mel-no-renderer" : 42,
}

class MelDataEditorOperator(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.simple_operator"
    bl_label = "Mel Custom Data Editor"

    custom_data : bpy.props.StringProperty()

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def addCustomData(self, context, keyStr, val = 1):
        for ob in context.selected_objects:
            self.addTo(ob, keyStr, val)

    def removeCustomData(self, context, keyStr):
        for ob in context.selected_objects:
            del ob[keyStr]

    def addTo(self, objet, keyStr, val = 1):
        objet[keyStr] = val

    def execute(self, context):
        if(self.custom_data not in default_data.keys()):
            print("invalid custom data key")
            return {'FINISHED'}

        if allSelectedHaveKey(self.custom_data, context):
            self.removeCustomData(context=context, keyStr=self.custom_data)
            return {'FINISHED'}

        self.addCustomData(context=context, keyStr=self.custom_data, val=default_data[self.custom_data])
        return {'FINISHED'}

class ProtoPanel:
    bl_category = "Mel"
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_options = {"DEFAULT_CLOSED"}

class EXAMPLE_PT_panel_1(ProtoPanel, bpy.types.Panel):
    bl_label = "Mel EZ Custom Prop Editor"

    def getSharedKeys(self, context):
        result = []
        for key in default_data.keys():
            if allSelectedHaveKey(key, context): 
                result.append(key)
        return result
 
    def showSharedKeys(self, context):
        layout = self.layout
        shared = self.getSharedKeys(context)
        layout.label(text="Shared keys: ")
        for sharedKey in shared:
            layout.label(text=sharedKey)

    def drawOperatorFor(self, keyStr, context):
        layout = self.layout
        allHave = allSelectedHaveKey(keyStr, context)
        labelText = ''.join(["REMOVE" if allHave else "ADD", " ", keyStr ])
        layout.operator('object.simple_operator', text=labelText).custom_data = keyStr

    def draw(self, context):
        for key in default_data.keys():
            self.drawOperatorFor(key, context)
        self.showSharedKeys(context)

classes = (EXAMPLE_PT_panel_1, MelDataEditorOperator)

def register():
    for cls in classes:
        bpy.utils.register_class(cls)


def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)

if __name__ == "__main__":
    register()