Accept variables in @Requires and requires.filter about ipopo

[tcalmant]

A new handler will be defined, e.g. @RequiresVarFilter, in order to avoid losses of performance in @Requires and other existing handlers.

The filter will be parsed when the handler starts and when component properties are updated.
The variables will be injected in the filter string using the string.Template class.

[tcalmant]

The first implementation of this handler is live.
The variables are injected using str.format instead of the string.Template class.
Some tests have been written for the "single dependency" mode; the "aggregated dependencies" tests will come later.

Note: the implementation isn't atomic: each requirement and each injected service are handled one by one.

[tcalmant]

Tests fully implemented after 848b8de: the implementation of RequiresVarFilter can now be considered stable.