[FEATURE] Scoped variables

[stefanr]

Short intro

• Extended the f:variable view helper to support a new backwards compatible scope attribute
  • Value: "global" (default) or "local"
• When f:variable is used with scope="local" and is placed within a view helper opening a new variable scope via ScopedVariableProvider, the variable is only added to the local variable provider
• Added a new f:scope view helper to simplify the use of local variables
  • It's actually similar to the f:alias view helper but without the need to initially declare variables

Example

<f:variable name="foo" value="World" />
<f:scope variables="{foo: 'Fluid', bar: 'Lorem'}">
  <f:variable name="baz" scope="local" value="Ipsum" />
  {bar} {baz} {foo}!
</f:scope>
Hello {foo}!

Output:

Lorem Ipsum Fluid!
Hello World!

After the scope {foo} is restored, {bar} and {baz} is removed.

[mbrodala]

What is the exact usecase which cannot implemented with the current state?