Add a FoldingRegionProvider that folds on each { and } pairs

The DefaultFoldingRegionProvider creates folding regions based on the AST structure.

It should be useful to create a new FoldingRegionProvider that creates folding regions considering the opening and closing curly braces.

Steps to reproduce:

1. Create a new Xtext project with the following grammar:

grammar org.xtext.example.mydsl.MyDsl with org.eclipse.xtext.common.Terminals

generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"

Model:
	greetings+=Greeting*;

Greeting:
	'Hello' '{'
		'from' sender=ID 'to'
		'{' receivers += ID+ '}'
	'}';

2. Start runtime Eclipse and create a test.mydsl file with the following content:

Hello {
	from Tamas to {
		Eclipse
		Xtext
	}
}

3. Xtext calculates only one folding region on default (left side). Using the improved FoldingRegionProvider, two folding regions should be calculated (right side)

Start with org.eclipse.xtext.ui/src/org/eclipse/xtext/ui/editor/folding/DefaultFoldingRegionProvider.java and compare its AST-based folding behavior with the brace pairs in the issue's MyDsl grammar. Reproduce the example in a runtime Eclipse instance, then verify that the nested grammar produces two folding regions on the right side.