Implementation of multimedia editor.

This commit contains the changes by @qutorial and @bibstha that
implement a more sophisticated card editor, which includes the ability
to import photos and audio directly from the device.

This is squashed down version of a number of changes that were made,
with my own edits to remove changes that are not related to the feature
itself.
Merge pull request #40 from flerda/v2.1-multimedia-editor

Implementation of multimedia editor.
Bumped version to 2.1alpha4
Merge remote-tracking branch 'ankidroid/v2.1-dev' into v2.1-dev-ospalh-special
My change to use html5
Bring the style in line with AnkiDesktop