GtkAda contributions 2.2

This library extends GtkAda, an Ada bindings to GTK+ (GIMP) toolkit. It addresses the following issues. Tasking support; Custom models for tree view widget; Custom cell renderers for tree view widget; Multi-columned derived model; Extension derived model (to add columns to an existing model); Abstract caching model for directory-like data; Tree view and list view widgets for navigational browsing of abstract caching models; File system navigation widgets with wildcard filtering; Resource styles; Capturing resources of a widget; Embeddable images; Some missing subprograms and bug fixes; Measurement unit selection widget and dialogs; Improved hue-luminance-saturation color model; Simplified image buttons and buttons customizable by style properties; Controlled Ada types for GTK+ strong and weak references; Simplified means to create lists of strings.