Fixed a bug that was eventually making pgModeler enters on a endless
looping when calculating the chained operations length.

Created a class called ColorPickerWidget to handle operations where the
user need to configure colors for objects (pending implementation).

Added an attribute to relationship class to control custom line color.
A significant code optimization was made on TagWidget by changing the
tool buttons that was handling color configuration by color pickers.

Added a color picker on RelationshipWidget to configure custom line
color.
Merge branch 'custom-rel-line-color' into faithful-elephant
Optimized the code on  AppearanceConfigWidget by adding color picker in
order to select color of elements.
Merge branch 'appearance-cfg-change' into faithful-elephant