Proto Model Header Data Mutation

[RobertBColton]

This is an alternative to #166 that adds the ability to mutate the header data returned by the base proto model. I use it to give the column headers of the path editor's points list special icons. I hid the vertical header of the path points list table too because we don't actually want it and it started showing row numbers now that my implementation allows the base QAbstractItemModel::headerData to return the section number as a string for Qt::DisplayRole role. We can also use this later to translate the column headers displaying field names for i18n.

I was also thinking that this could be static and fundies did mention I could just use the proto type name so that messages still get unique header data. However, I don't know what other kind of dynamic crap we might want to do so I decided not to make it static yet. Static doesn't exactly work with things like sorting and other transformations of the data.

[fundies]

Perhaps its better to just store the string in the QVariant as I think those can be cast to icon or pixmap or whatever with QVariant.as<QIcon>()

[RobertBColton]

I didn't do that fundies because I want to be able to set the tooltip and stuff too. In the room layers view I'm just going to show eyeball icon for layer visiblity with a "Visible" tooltip.