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This fixes a possible exception slicing problem and simplifies the code by using a shared_ptr to close the nifti_image* we read from.
Also, the "header" variable isn't closed properly, which is fixed by using only one nifti_image to read from instead of two.

[ADD #176] - the network editor now also accepts drops properly and forwards the handling of these events to the main window.

[ADD] networkEditor now opens the standard add-module menu when pressing middle mouse button or by double clicking while pressing ctrl or shift. Additionally improved the filter to interpret the filter text in a tokenized way (space is delimiter).

[ADD #175] - this is a filter mechanism for our module context menu. When the menu is opened, type your text and the menu will contain only the matching modules. Pressing escape resets the filter. Closing a menu resets the filter too.