Copy experimental agda syntax file (see https://git.reviewboard.kde.org/r/117167/).
Update cabal.
Mention Agda in readme.
Add missing source file.
Sync with upstream syntax file (ktexteditor/src/syntax/data/agda.xml).
Merge pull request #52 from mcmtroffaes/feature/agda-syntax

Merge upstream agda syntax file from ktexteditor.