Fix PHP Fatal from unchecked getStorage()

Additionally implemented a slightly more optimized approach.
Polished pull request as requested

Fixed formatting and removed a left over commented echo
Removes Impress styles from appinfo/app.php.
[Media] Improve tag extraction
[Media] Put those stripslashes back
[Media] Fix copy/paste in phpdoc
Highlight current sort order in Tasks app

This fixes owncloud/apps#823 .

The button of the current sort order is now switched to light blue
(similarly to how the Calendar app does it).
[Media] Update extraction flow
[Media] Add interface to classpath
Fix images path
Fix missing token information for settings
Update version
Update info.xml

Change required version of ownCloud and add contributor.
Use OCP namespace
Use img instead of css to display background image

With css, the background image is "cut" by other div (for example, you
may not see the background image in the files list if there is too many
file rows).
I use an img, with position absolute, bottom right like the css
background and small opacity to prevent visibility problems.
Update version
Use OCP Namespace in appinfo/app.php
Fix img path with OCP\image_path

Thx to @eMerzh for pointing it
Change indentation spaces to tabs
Remove unneeded and broken require