Dynamic print frame: provide possibility to cancel a running print

[astrid_emde]

if a print takes too long and the result is not given back, you can't cancel the printing

for more information about the process of the dynamic print have a look at #455

[verenadiewald]

for 2.7: ​http://trac.osgeo.org/mapbender/changeset/7926[[BR]] while print is running, there is no possibility to stop the server action -> to have a bit of control you can now set the element var timeout, so you can define the maximum time a print action can take (reaching the defined timeout will stop the ajax request)

[verenadiewald]

for 2.7: ​http://trac.osgeo.org/mapbender/changeset/7926

for trunk: ​http://trac.osgeo.org/mapbender/changeset/7927