Merge branch 'develop'
Important change: XMLParser class is not a singleton anymore.

Introduced a method DatabaseModel::getXMLParser() in order to return the
xmlparser of the model to permit user create new objects from xml code
within the current database model.
Fixed a regression related to special objects recreation after rename a
pk column. The problem was that the relationship was wrongly storing the
old name of the columns at connection time.
Important change on SchemaParser class not being a singleton anymore.

Added the static methods setPgSQLVersion and getPgSQLVersion on
BaseObject to override the PostgreSQL version used to generate code for
all instances of BaseObject.
Change dialog changed to a floating widget.

Minor layout adjustment on update notifier widget.
Merge branch 'about-dlg-change' into faithful-elephant
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
Minor model objects style refreshment

UI style improvement in progress
Minor adjustments on ui-style.conf

Dropped the option "-no-stylesheet" from Application class.
Revert back to normal operation the update notifier.
Finished the UI refreshments
Merge branch 'ui-style-improvement' into faithful-elephant
Reletionships 1:1, 1:n, fk now supports a visual link between tables
through their fk colums.
Created a more elaborated central widget with some basic operations like
open, create and load recent models.
Merge branch 'rel-conn-improvement' into faithful-elephant
Minor improvement on custom relationship colors.