Add IconImageView and IconImageButton

[1zaman]

Although single icons can be rendered as text in text views, it requires setting a fixed size. Using a drawable allows it to automatically scale according to the size of the widget. Therefore, this pull request introduces IconImageView and IconImageButton to enable using the drawable in XML. They also define custom attributes for the icon properties, which improves readability and extendibility. Furthermore, extending ImageView and ImageButton provides easy inoperability with other drawables (i.e when switching between XML drawable resources and IconDrawable).

Resolves #132