Minor cleanup

Fix GTK warning assigning null value to entry
Fix disposal of placeholder layout
Clean up some source analysis warnings
Add PasswordEntry.{PlaceholderText,SecurePassword}
Fix bad cast in Gtk PasswordEntryBackend
Merge pull request #241 from steffenWi/master

[Wpf] Update CellUtil.cs to support CheckBox in ListView
Don't use spaces for indenting
Improved ListView sample
Add Activated events to TextEntry, PasswordEntry

GTK# only for now
Fix PasswwordEntry.Activated
[GTK] Missing features

Implemented GetBackendForContext.
Implemented ImageHandler.LoadFromFile using the Pixbuf constructor
since PixbufLoader doesn't support some formats such as icns.
Merge branch 'master' of https://github.com/mono/xwt

Conflicts:
	Samples/Samples/ListView2.cs
	Xwt.WPF/Xwt.WPFBackend.Utilities/CellUtil.cs