Refactors window creation

The class AbstractWindow provides attributes and methods commonly
used by windows in this application, extends JFrame and should
be extended itself when creating new windows for ASxcel.
Refactors dialogs

The creation of dialogs has been removed from the View and added
to the AbstractWindow which now has methods to display simple
dialogs. These dialogs open above the according instance of
AbstractWindow and are easy to use and to maintain.
Adds comment in MenuBar

The private attribute 'listener' was missing a comment, which gets
addes with this commit.
Removes error message in case of SQLException

Because a real exception handling is absolutely necessary the
pseudo error message that was displayed when a SQLException was
triggered somewhere gets removed.
Adds comment in BooleanCellRenderer

The class description was missing and gets added with this commit.
Improves processing of changes to table data

If data is changed by the user that is specific to a single route
only a change event for his single row is triggered, and not for
the whole table like it is the case for the rest of the data.