Add mount configuration priority information
Clarify priority numbers and improve default priority info
Change wording of priorities
Merge pull request #342 from owncloud/files_external_priority

Priority on external storage mounts
fix some dev docs
add icons docs
add info about background repeat and position
use png instead of gif
remove p images
convert svg to png
add docs on settings area
add slide toggle docs, owncloud/core#8701
better slide toggle description