Refactor MainApp.vue to use the Composition API

Pull request checklist

Please check if your PR fulfills the following requirements:

[ ] You can build successfully the whole project with your changes locally
[x] You use the coding conventions and the naming conventions of the project
[x] You use tabs for indentation
[x] You make sure that your PR is not a duplicate
[x] This PR is ready to be reviewed and merged
[x] This PR merges into the development branch
[x] There is no TODO in the code
[x] There is no spelling or grammatical errors in the code
[x] Details of the issue or feature are documented below
[x] This PR is not a breaking change (e.g. changes that would cause existing functionality to change)

Français `fr`
Utilisation de la Composition API.

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