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.

Well, this does not make a copy but it's just creating an alias. This means you're modifying the object in-place by adding the fake properties and then later removing them again. This is not a workable approach since these fake properties will not be distinguishable from its real properties by the rest of the code. Just imagine we would clone the selected object, it would get cloned together with its fake properties.

Fortunately I think it's not that hard to solve. You just have to make a value-copy of the current object's properties before adding the fake properties to it. So starting with something like: `mCombinedProperties = mObject->properties()`.