made path in sly_I18N constructor optional

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branch : 0.8
extra : source : 294fa65a5c21d634948cdb45d0a1c94521cd9f22
if labels and stuff is translated for form elements, so should the label

--HG--
branch : 0.8
fixed missing placeholder

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branch : 0.8
catch broken module inputs and return a sane fallback value (that's why we have this helper, to avoid this clutter in modules)

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branch : 0.8
moved setting the sly-form-select class into the constructor

--HG--
branch : 0.8
hello RC2

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branch : 0.8
back to dev

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branch : 0.8