Re-introduces custom image base layers for OpenLayers

This has initially been committed to gerrit.wikimedia.org on Oct 18, 2012 02:47
See https://gerrit.wikimedia.org/r/#/c/28430/

This feature has been part of some old version of Maps already but got broken a long time ago. This fixes all issues and re-introduces the original feature plus various improvements to Maps. Custom layers have to be defined in a Layer namespace. All layers will be saved in the database, therefore running update.php is required for this to work.
This feature is still rather experimental in this commit.
Merge branch 'master' into danwe_custom-maps

Conflicts:
	Maps.php
	Maps_Settings.php
	includes/criteria/CriterionMapLayer.php
	includes/layers/Maps_ImageLayer.php
	includes/layers/Maps_KMLLayer.php
	includes/services/OpenLayers/CriterionOLLayer.php
	includes/services/OpenLayers/Maps_OpenLayers.php
	includes/services/OpenLayers/Maps_ParamOLLayers.php
	includes/services/OpenLayers/jquery.openlayers.js
Merge branch 'master' into danwe_custom-maps

Conflicts:
	Maps.classes.php
	Maps.i18n.php
	Maps.php
	includes/Maps_LayerPage.php
	includes/Maps_Layers.php
Replace "@since dw1" with "@since 3.0"

Getting ready for integrating work on the custom maps feature back into master.
Updated INSTALL.md and RELEASE-NOTES.md about custom image layers feature
Merge pull request #1 from Danweetz/danwe_custom-maps

Reintroduction of custom maps feature