Hide content of binary files (fixed #459)

- Don't show content of binary files (fixed #459): also have a button to let people edit them anyway and have a preferences to let people always edit some files types as text.
- Remove useless load when showing an image.
- Fixed styling of checkbox in the setting dialog (input -> input[type=text]).
- Made focus ring of toolbar buttons (and other spark-button-s) more beautiful
- Improved dialogs consistency
- Remove bezel around the close buttons of the dialogs

@devoncarew