Add assign colors operator and UI.

• Add an "assign colors" operator that associates with almost any entity type and assigns all the colors it is provided to the entities in round-robin fashion. Colors are specified using an extensible StringItem that holds HTML-formatted hexadecimal RGB (or RGBA) color specifiers. If no colors are provided, then the entities are marked with the "invalid" color so they will inherit the default color.
• Add a new library, smtkPQOperatorViewsExt to hold custom ParaView operator views.
• Add a new operator view for "assign colors" that uses ParaView's color and color palette choosers to populate the "colors" parameter of the operator with values. The view also uses Qt's settings object to remember the last color and palette chosen by the user across sessions.
• This also fixes several issues discovered while debugging "assign colors."