fix #271 (Search for empty values should be allowed)
add cancel button
check if cancelled in the GeneralText Exporter
notify user when export was cancelled
only show the "cancel" button when exporter supports cancel action
allow to abort some of the exporters (fixes #269)
Always create a corpus.properties file, also in the case the user has not
defined one.

closes #272
Wrote comments.
Throw also an io exception when the file traverser does not find any corpus.

closes #273
don't ignore exceptions in the console output
create subdirectories on zip import if necessary
artificial marker was not set properly
deactivate TransitivePrecedenceOptimizer temporary for benchmarks
tests needed to be adjusted if precedence optimizer is disabled
don't join on node_anno_ref if node annotation definitions (thus namespace and name) are not the same
sort automatically generated frequency query entries by the node variable
update to vaadin 7.1.11
better handling of "OR" in the frequency query automatic query generator
use the provided database.properties file in Kickstarter when initializing the database
don't ignore error if exception list is empty