Update translation demo project to include changes from GSoC 2020 i18n

[SkyLucilfer]

NOTE: This is for Godot 4.0

As part of my GSoC work on i18n, I have updated the translation demo project to incorporate the newly implemented features (CSV plurals, PO plurals, POT generation etc.)

I'm making a pull request first for mentors to review. They should have other plans.

[aaronfranke]

@SkyLucilfer Soon we will begin porting the demos to the current master branch in preparation for Godot 4 alpha. Can you update this PR so that it works with the current master branch of Godot, and check if there is anything that needs doing before merging?

[aaronfranke]

@SkyLucilfer Can you rebase this PR and ensure it works with the latest master branch of Godot?

[Anutrix]

@SkyLucilfer Can you please rebase this PR?