[GTK] fix Util.RenderPlaceholderText
* works for Gtk.Entry and Gtk.TextView now
[GTK] optimize Gtk.Justification conversion readability
[GTK] add a custom GtkMultilineTextEntry widget
* handle sizing using overloads
[GTK] ignore new lines for single line mode in TextEntryMultiLineBackend
[GTK] raise OnActivated for multiline entry in single line mode only
[GTK] let Gtk handle the sizing for TextEntryMultiLineBackend
* mimic Gtk.Entry width behavior (behave like: https://git.gnome.org/browse/gtk+/tree/gtk/gtkentry.c?id=2.24.24#n2904)
[GTK] clean up TextEntryMultiLineBackend.cs