Visual feedback for multiple object properties

Here's my implementation for some visual feedback when manipulating multiple object properties.

```
obj1:
    a = 1

obj2:
    a = 2
```

Having both of these objects in your selection will result in having the current objects 'a' property being grayed out but still allowed to be changed.

```
obj1: <- current object
    a = 0
obj2:
    b = 1
```

Having both of these objects in your selection will create a 'fake' property 'b' in obj1. This fake property will be grayed out and will be removed after changing the selection if the fake property was not set.

This request is mostly for feedback regarding the implementation at the moment. In order to get this to work I had to modify the qtpropertybrowser code to support arbitrary colors. But I'm not sure if that is the best solution. I don't know how Qt handles things like color schemes if that even is an issue.

This is the wrong place for updating the color. The color updating should be based on changes to the value of the object, rather than edits made in the property editor. This is because changes to the values may happen also from other places, most commonly the undo system, and they need to trigger the color updating as well.

So, `updatePropertyColor` or `updateCustomProperties` should be called from for example `propertyAdded`, `propertyChanged`, `propertiesChanged` and even `propertyRemoved`. It needs to be done when any of these changes are made to any of the selected objects, not just when they happen to the current object.