MainWindow determines the active Icon and emits the corresponding delete signal
Updated the UML to reflect the flow of actions and operations
I added isSelected() to Icon for filtering Icons by IconClassification

It traverses the classification tree upwards for every IconClassification
assigned to the Icon instance.
I changed selected flag to true by default

Moreover I restructured a few lines in the data() function of
IconListModel for better readability. No functional change has been
applied though.
Added IconFilterProxyModel to apply selection filters to icons

I utilize QAbstractProxyFilterModel to achieve the filtering in between
IconListModel and IconSetOperator.
More info about QAbstractProxyFilterModel can be found here:
http://qt-project.org/doc/qt-4.8/qabstractproxymodel.html
Added getIconSet() to IconListModel

This is necessary so that IconFilterProxyModel can address the underlying
data of its source model to check whether or not an icon is to be displayed.
Incorporated IconFilterProxyModel in the existing class hierarchy

IconFilterProxyModel is now transparently used in place of IconListModel.
Notify IconSetObservers, when filter has been applied via selection checkbox
Merge branch 'filter'

Filtering Icons by the checkboxes on the left works now!
Merge branch 'master' of github.com:mank319/Icon-Set-Forge