Update to 2.18.3

Changelog:
  1. What's new in Version 2.18 'Las Palmas'?
  ===========================================

This release has following new features:

- Symbology: Color picker is now embedded in layer style panel
- Labelling: Substitution list support for labeling
- Labelling: Improved line label placement algorithm
- Labelling: Label polygons using curved labels along perimeter
- Data Management: Add flag to only copy selected features
- Forms and Widgets: Allow controlling labels for individual edit widgets
- Forms and Widgets: Conditional visibility for tabs and group boxes
- Forms and Widgets: Client side default field values
- Map Composer: True North Arrows
- Processing: Point on surface algorithm  added
- Processing: New algorithm for geometry boundary
- Processing: New algorithm for calculating feature bounding box
- Processing: Processing dissolve algorithm accepts multiple fields
- Processing: Optimised processing clip algorithm
- Processing: New algorithm for merging connected lines
- General: Automatic links in identify results
- General: Mouse wheel over color dialog sliders
- General: Add custom color schemes to color button drop down menu
- Data Providers: Preview for WMTS + added XYZ tile layers
- QGIS Server: Possibility to segmentize feature info geometry in server
- Plugins: DB Manager: Add the ability to update SQL Layer
- Programmability: New expression functions
- Programmability: Expose GEOS linear referencing function to QgsGeometry

(ryoon)