Update internal libgadu to 1.12.0

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branch : release-2.x.y
Better check for libgadu GPL compliance, some tidying up

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branch : release-2.x.y
Gadu-Gadu: revert things accidentally removed, fix build

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branch : release-2.x.y
German translation update

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branch : release-2.x.y
Allow incoming IQ stanzas with an empty 'from' if they're in response to
an outgoing stanza to our bare or full JID. Patch from Thijs Alkemade
from https://developer.pidgin.im/ticket/15879

Fixes #15879

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branch : release-2.x.y