Delete Gtk3.pm

Not needed here anymore.
Added GtkComboBoxText example
Merge branch 'master' of https://github.com/dave-theunsub/gtk3-perl-demos
fix to Gtk3::Dialog
Added GtkRadioButton demo
Added GtkIconView example
removed unnecessary variable
Add patches from Thierry for builder.pl
remove constant
Update README.md
Added GtkStack Demo
Added GtkFlowBox demo