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.

Since `updateCustomProperties` deletes and re-creates all properties, the previous code that carefully updated only the changed property makes no sense here anymore. Preferably though, the property should just be added on-demand only like it used to be (and adding it to `mCombinedProperties` when necessary), and in addition its color needs to be updated.