Changelog for QGIS 2.12.0
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* General : New welcome screen

* General : New expression functions in 2.12

* General : Mutually exclusive layer tree groups

* General : Filtering for field values in expression widget

* General : Advanced settings editor

* General : Ongoing improvements to code quality

* General : User Interface Theme support

* General : Variables in expressions

* Symbology : Export thumbnails from style manager

* Symbology : New option for limiting size in mm when using map unit sizes

* Symbology : All color ramps can now be edited

* Symbology : Improvements to displacement renderer

* Symbology : Improved handling of SVG marker outlines

* Symbology : Add pixels as option for all symbology size unit choices

* Labelling : Control priority of labeling obstacles

* Labelling : Draw only labels which fit inside polygons

* Labelling : Data defined control over label priority

* Labelling : Rule-based labeling

* Labelling : Data defined quadrant when in "around point" mode

* Labelling : New options to control how polygon layers act as obstacles

* Labelling : Option for obstacle-only layers

* Print Layouts : Atlas navigation improvements

* Print Layouts : Custom format for grid annotations

* Print Layouts : Multiline text handling and automatic text wrapping in composer attribute tables

* Print Layouts : Advanced customisation of cell background color 

* Print Layouts : Option to hide pages from view/export 

* Print Layouts : Force vector layers to render as a raster images

* Print Layouts : Add fit page to contents option and options for cropping exports to contents

* Print Layouts : Data defined control over map layers and style presets

* Digitising : Digitising improvements

* Data Management : DBManager Improvements

* Data Management : Conditional formatting for attribute table cells

* Data Management : Support for relative paths in widgets 

* Analysis Tools : Added number of vertices to derived fields in identify tool

* Analysis Tools : Geometry Checker and Geometry Snapper plugins

* Analysis Tools : Raster alignment tool

* Application and Project Options : Encrypted password management

* Browser : Improvements to PostGIS connections in browser

* Data Providers : PostGIS provider improvements

* QGIS Server : QGIS Server Python API

* QGIS Server : getMap in dxf format

* Plugins : Update of the GRASS plugin

* Programmability : Maptools moved from app->gui

* Programmability : Open scripts in external editor

* Programmability : Editing layers via `with edit(layer):`

* Programmability : New classes for PyQGIS programs

* Programmability : New API for labeling engine (QgsLabelingEngineV2)