[Drawing] Fix possible infinite loop
Ensure Toolkit.Type is set when initialized with Application.Initialize* APIs
[Mac] Use the correct font for dialog buttons
rename PreviewTextInput event to TextInput

Xwt has no preview events. To stop an event, the Handled property
of the event arguments object should be used.
add null check for text in TextLayoutBackend

the text should never be null, because the toolkits require
a string value, when drawing. Otherwise WPF and MAC
throw a NullReferenceException and Gtk prints
a warning to the console (Pango-CRITICAL).
[Gtk] add TextInput event support

Handles the TextInserted event if the controlled Gtk Widget implements Gtk.IEditable.
Otherwise Gtk.IMMulticontext is used to get the typed text. Key recognition
and conversion is managed by the os specific input method in this case.
Merge branch 'sevoku/fix-textlayoutbackend-null-string'
Merge branch 'sevoku/text-input'

Conflicts:
	Xwt.Gtk/Xwt.GtkBackend/Gtk2Extensions.cs
	Xwt.Gtk/Xwt.GtkBackend/Gtk3Extensions.cs
	Xwt/Xwt.csproj