WFS: get total number of features when maxFeatures is hit

When a WFS filter (plugins GetFeature and QueryBuilder) returns too many results (nb > maxFeatures), the list of features is truncated to the first <maxFeatures> results.

With this PR, when the max number of features is reached, an additional WFS GetFeature request with ``resultType="hits"`` is triggered to get the actual number of features matching the filter (requires WFS 1.1.0). The result is then displayed in the bottom toolbar of the FeaturesGrid (FeaturesWindow does not displayed the number of features).