Squashed 'core/' changes from 7dacfd8..9e24e5c

9e24e5c Bugfixes and singular field values

git-subtree-dir: core
git-subtree-split: 9e24e5cd51bd4acdf55ddee5db6c19d02c1385ba
Merge commit 'a75c590e487043022275c68754dfe3d7a6a3ec5e' into develop
Use singular parameter

Dropbox invalidates an setup token once it is used for the first time.
Therefore a user has to generate it again every time during setup when
he's revising the plugin parameters.