QMargins: plaster API with Q_DECL_NOTHROW

This is mostly straight-forward, but some things are worth noting:

1. Yes, this is necessary. The noexcept operator looks for noexcept tagging,
   not at the contents of the function to determine whether to return true.
   The more conditionally-noexcept functions are used, the more important it
   becomes that low-level classes are correctly marked noexcept. In that, it
   is like constexpr.
2. In accordance with the rules governing noexcept specifications for the
   standard library itself, the operator/-family of functions are not marked
   as noexcept, since they have preconditions and thus a narrow contract.
   Narrow-contract functions should not be noexcept. All other functions
   have wide contracts (ie. no preconditions).

Change-Id: I2cb1f951a92dcb25eac4d9afc5b7780311e39492
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

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