[Qgis-psc] Bugfixing for QGIS 2.16

[Matthias Kuhn]

Dear PSC,

here is a list of what has been done as part of the sponsored bugfixing
period.
I didn't focus on a particular area of the code nor did I count each
hour spent on bugfixing.

Besides this, I spent as usual time on maintaining the continuous
integration platform (travis) and pull request reviewing/merging and
followed up a couple of tickets which I was unable to solve in the end.

Thanks a lot for sponsoring this effort and have fun with a stable QGIS 2.16

Matthias

---------------------------------------

Improve QgsMapLayerRegistry stability
  https://hub.qgis.org/issues/14822

Processing: Difference algorithm: don't ignore invalid geometries by default
  https://hub.qgis.org/issues/9297

Fixed some Python 3 compatibility issues

Work on re-enabling osx tests (merged after 2.16 release)

API documentation

startup.py is executed twice
  https://hub.qgis.org/issues/15189

UX fixes
 * Attribute table: always show sequential column header numbers
 * Relation Reference Widget: Only enable form button when a feature is set
 * Show feature id in attribute table tooltip
 * Initial widget focus in credential dialog
 * Default timeout for message bar items
 * "Add button" state in postgis source select dialog

Install missing header files

Crash on exit
  https://hub.qgis.org/issues/15088

Network cache directory
  https://hub.qgis.org/issues/15111
  https://hub.qgis.org/issues/14990

Feature form hidden behind main window on Windows
  https://hub.qgis.org/issues/15103

Offline editing: Respect relative paths

Offline editing, changed attributes end up in wrong feature
  https://hub.qgis.org/issues/14727

Offline editing: Don't crash with raster layers
  https://hub.qgis.org/issues/14848

Relation form grows infinitely
  https://hub.qgis.org/issues/14977

Fix crash when loading style in layer properties

Fix crash when closing docked attribute table
  https://hub.qgis.org/issues/14909
  https://hub.qgis.org/issues/15018

Fix default-misconfigured gpsbabel path
  https://hub.qgis.org/issues/14866

Show correct feature count with rule that requires geometry
  https://hub.qgis.org/issues/14758

Fix crash when using 2.5D renderer with incompatible layer
  https://hub.qgis.org/issues/14814