Support for Google Gemini

This release adds Google Gemini as a translation provider.

Please note that the Gemini API can only be accessed from IP addresses in certain geographic regions at the moment: https://ai.google.dev/available_regions

To use Gemini as a translator you will need an API key. I believe there is a simplified procedure for getting one at https://ai.google.dev/ if you are in a supported region. Alternatively you can set up a project on https://console.cloud.google.com/ and generate an API key. You must ensure that Generative AI is enabled for the project and the key.

Google have only exposed the Gemini 1.0 Pro model via the API at the moment, though you can sign on to a waiting list to access to the new 1.5 Pro. In my tests Gemini 1.0 Pro is comparable to or perhaps a little better than the latest GPT 3.5 model from OpenAI when it comes to subtitle translation, and is basically free to use for this purpose (https://ai.google.dev/pricing).

This will be a pre-release/beta version to encourage testing and exploration. Please drop a note in discussions if you are able to use Gemini and let us know how your experience was, or create an issue if you encounter any problems (other than Gemini not being available in your region).

Note that the readme and the wiki have been thoroughly revised to reflect the changes in v0.6, so if you have any questions it is probably worth checking those to see if they're addressed there.

Update: updated to the latest PySide6 version to hopefully fix some random crashes in the GUI
Update: added default models for each provider for the command line interface. Use -m "model name" to use a different model.