#270 remember the failed images

The images which failed loading are now also saved. Thus a reload of
those images is not necessary.
 #270 use a SwingX-utils method to scale the images

Use GraphicsUtilities.createThumbnail() instead of
Image.getScaledInstance(), as it should be faster.
 #270 avoid exception in scaleImage()

GraphicsUtilities.createThumbnail() throws an Exception if the size is
too big, therefore the old scale image can be used instead. Also nothing
is done if the image has already the right size.
 #270 small changes
 #270 load the bullet points once
 #270 moved the utils regarding the RestrictionLevel

The utils from Sb_stadtbildUtils regarding the RestrictionLevel was
moved to an own class Sb_RestrictionLevelUtils
 #270 moved the determination of the image to the utils

The determination of the bullet point image and the information about a
RestrictionLevel was moved to Sb_RestrictionLevelUtils.
 #270 added bullet point to Sb_stadtbildserieEditor
 #270 Load the bullet point in different thread
 #270 remove the temporary field tmp_restriction_level

Before saving a Stadtbildserie its extension attribute
tmp_restriction_level has to be set to null. Otherwise a serialization
error will occur.
 #270 added checkboxes to search windows

Removed the combobox for the Nutzungseinschränkungen and replaced them
with three checkboxes for the user restrictions.
 #270 the user can search for the signal light

The user can e.g. search for those stadtbildserien, which he can use
internally or externally. Therefore it must be checked which
nutzungseinschränkungen fall under that criteria.
 #270 added double checked locking

Double checked locking was added to
Sb_RestrictionLevelUtils.determineRestrictionLevelForStadtbildserie()
 #270 added click listeners to checkbox labels

In Sb_StadtbildWindowSearch the tree traffic light checkboxes consist of
a checkbox and a label, because they need an icon and Swing can not
handle that case. If the user clicks on a label its corresponding
checkbox should switch its selection state.
 #270 comments
 #270 added a method to remove a entry for failed image

In Sb_stadtbildUtils a method was added to remove a bildnummer from the
failed image set, thus it can be loaded again. Furthermore similar
methods were renamed.
 #270 added possibility to clear the last shown image

In Sb_AbstractPictureGridObject a method was added with clears the
information about the last shown image. And a typo in that field was
corrected.
 #270 reload failed image

If a grid object is selected and the currently shown image could not be
loaded correctly it is reloaded.
 #270 select the green traffic light as default

In Sb_StadtbildWindowSearch the search should search for Serien which
can be used internally and externally.