Fix overlay + layout auto resize

- Added close button
- Overlay was preventing close button to be clicked.
- Auto zoom (unprefixed) enabled when image w/h ratio doesn't match div
w/h ratio
Code style
Before / After : improved CSS
Before / After : properly styled the close button
Merge branch 'master' into before-after-wip

Conflicts:
	app/styles/appimages.css.less
Merge pull request #74 from tobozo/before-after-wip

Fix overlay + layout auto resize