Clean unused data

[pionl]

Current WIP branch

• Delete unused images in /data/images/
• Delete unused sounds in /data/cs-sounds , /data/sk-sounds , /data/pl-sounds, /data/uk-sounds and /data/cs-alphabet , /data/sk-alphabet , /data/uk-alphabet , /data/pl-alphabet

Both should be runnable as tasks independently (like now we have build:dictionary and build:alphabet defined in package.json), so later we can decide if we run cleaning task for dictionary and cleaning taks for alphabet automatically every time or just manually once in a while.

It would be nice if we can run also it in some debug mode, when it just shows which files could be deleted but do not delete them.