Move generic image tools to Applications module

[schuhschuh]

I would suggest moving the following tools to the Applications module while renaming and possibly extending them as follows:

• calculate-filtering -> calculate-local-image-statistics.
• calculate-gradients -> calculate-image-gradients, add option to use finite differences instead.
• fill-holes -> fill-image-holes, add option to use NN as in fill-holes-nn-based instead of separate command.
  ...

[schuhschuh]

Other commands that are not generic enough or not yet moved to the Applications module should use draw-em- as name prefix to avoid potential conflict with other MIRTK Packages and to clarify which package this binary comes from. Maybe this should be a per default prefix prepended by the MIRTK CMake function mirtk_add_executable.