Fix custom link label being double HTML-encoded on each save

[boo-code]

Questions	Answers
Description?	When editing a custom link (BO > ps_mainmenu config > Custom links), getAddLinkFieldsValues() decoded the link URL with Tools::htmlentitiesDecodeUTF8() before pre-filling the edit form, but left the label untouched. Ps_MenuTopLinks::getLinkLang() returns both the link and the label HTML-entity encoded (via Tools::safeOutput()), so the label was pre-filled already encoded and then re-encoded on save, stacking the entities on every edit/save cycle (À → À → À → …) until the label became unreadable in the BO and the front office. The fix decodes the label exactly the same way the link is decoded.
Type?	bug fix
BC breaks?	no
Deprecations?	no
Fixed ticket?	Fixes #89.
How to test?	Add a custom link with a label containing a special character (e.g. À propos), save, then edit and save it again a few times without changing anything. Before: the label in ps_linksmenutop_lang.label, in the link list and in the edit field gains an extra layer of HTML entities on each save. After: the label keeps its original value across saves. Verified against the database: editing a link whose stored label is À propos now pre-fills the form with À propos instead of À propos, so re-saving leaves it unchanged.
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Root cause spotted by @Daaaaad in the issue — getAddLinkFieldsValues() decoded $link['link'] but not $link['label'].

[ps-jarvis]

Hello @boo-code!

This is your first pull request on ps_mainmenu repository of the PrestaShop project.

Thank you, and welcome to this Open Source community!