add TextEntry.PlaceholderText samples with different alignments
[WPF] fix PlaceholderText alignment and initialization
[GTK2] convert RenderPlaceholderText to an extension method
and move it to Gtk2Extensions
[GTK2] add Gtk.Widget.RenderPlaceholderText_internal
extension method for easier implementations for
other editable widgets.
[GTK2] fix TextEntry.PlaceholderText positioning
for different alignments
add PasswordEntry.PlaceholderText sample
Merge pull request #373 from sevoku/fix-wpf-gtk-placeholdertext-rebase

Fix PlaceholderText placement on WPF and GTK (rebase)