Show partials labels

This patch allows to show/hide labels that cross edge of the current extent.
The option can be switched into PAL global settings dialog (by default, show partials labels, as it was before)
QGIS Mapserver can read this setting from project file. Changing it from request is not implemeted yet. Unittests and bindings are included.
For QGIS Mapserver can occur cutting labels by right of the screen due to bug:  http://hub.qgis.org/issues/8068