ECMS-5764: Slowness in UIJCRExplorerPortlet.processRender within ECMS Read document context

Problem analysis
	- Slowness in UIJCRExplorerPortlet.processRender divided into slowness in UIJCRExplorerPortlet.showDocument and UIDocumentInfo process rendering

Fix description:
	- Get value of document title property instead of check existence first because "exo:title" have value in most of cases
	- Improve UIJCRExplorer.getChildrenList(): Only return child nodes for current page
	- Improve UIJCRExplorer.refreshExplorer(): Call UITreeExplorer.buildTree() just if explorer tab is current view of side bar
	- Move getActionList for context menu outside of UIJCRExplorer rendering