Correctly start Pocket Paint or Paintroid, depending on our build

Depending on the build (release or debug) either a debug version of Paintdroid (debug) or the store version of Pocket Paint is offered as download. The download is only offered if there is no version of Pocket Paint or Paintdroid installed.

- added download of debug version of Paintdroid to checkIfPocketPaintIsInstalled() in LookFragment.

- added POCKET_PAINT_DOWNLOAD_LINK_NIGHTLY to constants.

Issue #586 

test: https://jenkins.catrob.at/view/Catroid/job/Catroid-Multi-Job-Custom-Branch/332/