Tweak tolerances for CanvasPoint and ComposerPdfPoint PAL tests

.. to succeed on Ubuntu 14.04.1 LTS
Consider map rotation in rendering of horizontal and parallel labels

See http://hub.qgis.org/issues/11814

NOTE: the map rotation should likely be considered at LabelPlacement
      configuration rather than at rendering time

Raises pixel mismatch tolerance for background labels tests
populating browser icons in threads moved to QgsDataItem
expression: more comments and test to avoid differences between operators declaration and text values
Fix glitch on mouse-wheel zoom event

See http://hub.qgis.org/issues/11811
ziplayertest fix
Revert "Fix glitch on mouse-wheel zoom event"

This reverts commit 40e0d78b42474c698cc66e549bfb18d9867167b2.

Fixed the mouse wheel zoom regression but it broke proper rendering
of rotated maps...
[fix #11475] expressions: add left and right associavity for dump of binary operators

complete test for left associativity