Fix generation of unique ids in layouts

A duplicate id issue can happen in generated ids when dealing with "base" names ending with _[number].

Example:

• use a widget with name "foo" and generate a unique id for it (byt rendering it once for instance) -> "foo" will be returned
• render this widget again -> "foo_1" will be returned
• now render a widget with name "foo_1" -> "foo_1" will be returned

=> duplicate id issue