fix some annotations-in-text issues (#532)

## Description, Context and related Issue

There surfaced two issues when testing with the Open-Faust data which
might be connected to #460.
1. JSONification of the `local:getAnnotations` function was not
complete. The function returned a map object `plist` as string, causing
some Javascript function to fail.
2. `teitext:getLabel#2` would return a sequence of strings (under some
circumstances) which was not allowed by the return value of the function
signature.

<!--- This project only accepts pull requests related to open issues.
Please link to the issue here: -->
Refs #460


## Types of changes
- Bug fix (non-breaking change which fixes an issue)
- Improvement
- Refactoring

## Overview
- I have performed a self-review of my code, according to the [style
guide](https://github.com/Edirom/Edirom-Online/blob/develop/STYLE-GUIDE.md)
- I have read the
[CONTRIBUTING](https://github.com/Edirom/Edirom-Online/blob/develop/CONTRIBUTING.md)
document.
- All new and existing tests passed.