El gr

additions:
* added a new languages/el-GR/ folder for putting there the greek translation files
* copied all files from core/language/en-GB/ to languages/el-GR/ folder

modifications:
* editions/tw5.com/tiddlywiki.info: added el-GR option to language dropdown box
* translate all files in the languages/el-GR/ folder from english to greek.

Note: I used version 5.1.3 prerelease. I am not familiar with GitHub.