[GEOS-6790] Add support for GWC In-Memory caching for TileObjects

Conflicts:
	doc/en/user/source/community/index.rst
	src/community/pom.xml
	src/web/app/pom.xml
	src/web/gwc/src/main/java/org/geoserver/gwc/web/GWCSettingsPage.java
	src/web/gwc/src/test/java/org/geoserver/gwc/web/GWCSettingsPageTest.java