Connector export: create cropping job
Cropping: respect a stack's translation
Cropping: fix composite image creation

Due to (apparently) a bug in GraphicsMagic, one is not able to create a
composite image based on a blank image that was created in memory if the
composite parts are in certain formats. This commit works around this by
creating a canvas image that is built from the first composite part and
therefore has the exact same storage properties.
Cropping: fix coding style and typos
Connector export: export single images instread of multipage TIFFs
Connector export: add error handling
Connector export: add success message when finished
Connector export: respect treenode section in estimated size display
Connector export: have only one place for initial data in dialog
Connector export: write error log file when a connector can't be exported

Up to now, the whole export was canceled if one connector couldn't be exported.
This commit changes this behavior to write an error log instead. This log
contains a table with all connector IDs, HTTP error codes and URLS to image data
that couldn't be retrieved.
Export treenodes: add basic front-end infrastructure
Treenode/connector export: make dialog usable for both exports
Treenode/connector export: rename source module to make more code reusable

Both, treenode and connector export, will share most of the code. Because
connectors can be considered as a special kind of treenodes, it makes sense to
have most code in a module referring to treenodes.
Treenode/connector export: refactor to support both export types
Treenode exporter: implement missing exporter functions
Treenode exporter: create metadata file

In each neuron folder a file called 'metadata.csv' will be created. This file
contains a comma separated meta data line for every treenode, each containing
treenode id, parent id, #presynaptic sites, #postsynaptic sites, x, y, z