List of known bugs:
[done] * add ui for various preview sizes
[done] * thumbnail preview (100%zoom only in first step?)
[done] * finish full screen preview (ala Krita filters)
[done] * aspect ratio in preview viewport has to be set correctly
[done] * preview area has to support opacity 
[done] * selecting non-filter item in filter tree shows unchanged layer
[done] * turning off preview: it shows non-filtered active layer
[done] * preview is computed using gmic command, not using gmic preview command
* add support for panning in preview viewport
* implement progress bar for filters and preview
* separate included gmic code to stand-alone library and link dynamically 
* if the gmic filter outputs layer smaller than original, it supposed to be centered to canvas size, not bitblt(0,0,image->rect) 
* what to do with selections for small preview?
* sometimes preview blocks the UI because it takes too long  => progress bar
* resizing the preview has to work (Small, Tiny, Large, ...)
* resizing "input/output area/preview viewport" from filter settings tree
* support all preview output modes
* gmic debug output slows down filtering, make it optional
* support for zooming/panning preview
* preview area needs border and scrollbars
* tiny, small, normal, large behaves exactly same for now
* "Preview mode" only 1st ouput works 
* missing: Favorite presets group
* missing: button for expanding all folders
* missing: Available filters information 
* scrollbars for filter settings that are huge! currently text is horizontally downscaled and unreadable
* gmic is located under Layer -> Apply Gmic action, it supposed to be in Filters 

