[Finder] adds adapter selection/unselection capabilities
merged branch jfsimon/finder-adapter-selection (PR #7212)

This PR was squashed before being merged into the 2.2 branch (closes #7212).

Commits
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f40adbc [Finder] adds adapter selection/unselection capabilities

Discussion
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[Finder] adds adapter selection/unselection capabilities

As we have many issues with the native finder adapter, it would be good to:
*  permit selection of a particular adapter which will be choosed over the others until its unselection
*  permit unselection of adapters to get the finder select trhe best on (by priority)
*  have `PhpAdapter` selected by default

This PR adds 2 methods to the `Finder`:
*  `setAdapter($adapter)`: selects an adapter by its name
*  `useBestAdapter()`: tells the finder to select best adapter by their priority

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes

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by jfsimon at 2013-02-28T14:05:25Z

@fabpot I just applied all your good advices, there was no bad one :)