[GTK] Fix ComboBoxEntry issue

Always set the text column. Don't clear the ComboBoxEntry
on creation, since that destroys the default cell renderer.
[WPF] Fix AppendPath

The current path was appended, but not the already closed figures.
Start work on adding support for mnemonics
[WPF] don't fire Window.Shown event until window is loaded
Finish implementation of mnemonics for Buttons and MenuItems
Add Mac MenuItem of UseMnemonic