* moved the source languages out of assets/ into their own directory (#356)
* split build.gradle into several smaller files
* improved word frequency validation during build time
* slightly optimized dictionary loading speed using pre-calculated file size
* fixed a potential crash when loading invalid assets
* fixed dictionary loading progress starting at 100% then jumping to 0% when manually loading two dictionaries one after another
* documentation update
* fixed Bulgarian layout: moved 'ь' to 8-key
* added a migration for removing all Bulgarian words, since the digit sequences are no longer compatible with the new layout
* fixed incorrect text case of some words
* removed some nonsense words
* added new Bulgarian words
