Glimmer DSL for SWT 0.6.2 image, radio, checkbox, toggle, and arrow

Glimmer DSL for SWT 0.6.2 has been released, adding the following keywords to Glimmer:

• image: builds an SWT image object when outside the Glimmer DSL hierarchy, otherwise sets image property on a widget (this second part was already there before)
• radio: shortcut for building a button with :radio style
• checkbox: shortcut for building a button with :check style
• toggle: shortcut for building a button with :toggle style
• arrow: shortcut for building a button with :arrow style

Here are detailed changes:

- Set default margins on layouts (FilLayout, RowLayout, GridLayout, and any layout that responds to marginWidth and marginHeight)

- Have scrolled_composite autoset min width and min height based on parent size

- Add `radio`, `toggle`, `checkbox`, and `arrow` widgets as shortcuts to `button` widget with different styles

- Add parent_proxy method to WidgetProxy

- Add post_add_content hook method to WidgetProxy

- Add `image` keyword to create an ImageProxy and be able to scale it

- Fix issue with ImageProxy not being scalable before swt_image is called

Happy Glimmering!