Fix typo in Greek translation.

Closes #424
Update slovak locale

Closes #431
source-editor.js: add missing semicolon.
Update README.md.

Add title to the Google play image and use code syntax where applicable.
Whitespace normalization.

Use spaces consistently and trim trailing spaces.
Closes #425
Fix xml formatting in login.xml
Losslessly compress png files.

Savings approximately -110KB.
Closes #426