Fix !important declarations being overridden by non-important duplicates

[jafin]

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Description

In SetLonghand, a later non-important declaration for the same property would unconditionally replace an earlier !important one. Now the existing declaration is preserved when it is !important and the incoming one is not.

Fixes the issue where parsing style="padding: 20px !important; padding: 0"
would incorrectly yield "padding: 0" instead of keeping the important value.

Fixes #193

[FlorianRappl]

LGTM!