add TextEntry.PlaceholderText samples with different alignments
[WPF] fix PlaceholderText alignment and initialization
[GTK] convert RenderPlaceholderText to an extension method
and move it to GtkWorkarounds.cs
[GTK] add Gtk.Widget.RenderPlaceholderText_internal
extension method for easier implementations for
other editable widgets.
[GTK] fix TextEntry.PlaceholderText positioning
for different alignments
add PasswordEntry.PlaceholderText sample
add new TextArea widget for multi-line text input
 * mark TextEntry.MultiLine obsolete
    
Some toolkits (like Gtk) have different widgets for single- and multi-lined
text input. This makes it possible to implement multi line text enty backend
for those toolkits in a clean way.
[WPF] track API changes (add TextArea)
[GTK] add Xwt.Alignment to Gtk conversions
[GTK] track API changes
[GTK] add ITextAreaBackend impl.
using Gtk.TextView as backend
Merge branch 'add-textarea-widget'

Conflicts:
	TestApps/Samples/Samples/TextEntries.cs
	Xwt.Gtk/Xwt.Gtk.csproj
	Xwt.Gtk/Xwt.GtkBackend/PasswordEntryBackend.cs
	Xwt.Gtk/Xwt.GtkBackend/TextEntryBackend.cs
	Xwt.WPF/Xwt.WPFBackend/TextEntryBackend.cs
	Xwt/Xwt.csproj