Canvas image: add Efl.Canvas.Text.Factory + use in Ui.Text

Canvas image: add Efl.Canvas.Text.Factory + use in Ui.Text

This interface has a simple 'create' method to create Efl.Canvas.Object
given a key.
This is used higher-up in Ui.Text in the next commit.

Ui text: add ability to set item factories

Added API to set an item factory object.
This is similar to the previous item providers (that worked with
callbacks).

You instantiate a factory object and set it on the Ui.Text object.
Each factory implements the "create" method from
Efl.Canvas.Text.Item_Factory.

This also includes 3 public factories (Image, Emoticon and Fallback):
  - Image factory: creates images from added entries (key strings)
  - Emoticon factory: creates emoticons by querying the theme
  - Fallback: creates image, then falls back to emoticon

If no factory is set, then the fallback (internal) factory is used.

See the added "Ui.text Item Factory" test in elementary_test for an
example of usage.

@feature