The fact is, operators' label are nearly always verbs, while properties labels are nearly always nouns. So this should already solve many translations' problems regarding noun/verb confusion.
This commit also simplifies a bit i18n usage:
*Now IFACE_ and TIP_ macros (or there context versions, CTX_IFACE_/TIP_) are used nearly everywhere (with one exception, where code is a bit complex and needs to manually test whether ui/tip translations is allowed, so no need to redo it later through those macros).
*Also, those macros are now defined to NOP in case WITH_INTERNATIONAL is false, which avoid testing that define everywhere in code!
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