Resizes menu icons

The menu icon are a bit too big, so this commit provides menu icons
with a size of 16x16 instead of 24x24.
Implements update, prepares for gh-17

With this commit the UpdateManager gets added, a class responsible
for checking if the database is up-to-date, and to support it
several changes have been made to the application.
Implement test for UpdateManager

To verify the correct behavior of the update a test has been written
that performs an update from version 1 to 2.
Implements airports per enterprise, closes gh-17

This introduces the feature that airports are selected per
enterprise to reduce the size of available airports when
creating routes. Internally this is done by associating
airports and enterprises using a n:m relation.
Fixes bug in airport initialization

An error existed in a SQL statement that was processed during the
initialization of the model. More specificially a parameter in a
PreparedStatement didn't get used.
Merge branch 'enterprise_has_airport' into development

The feature is implemented and allows to reduce the amount of airports
listed when creating routes to those airports that where actually created
by this enterprise. If different enterprises are in use that exists and
operate in totally different regions only the airports in the respective
region will get displayed in the future.