Update check on startup

[Akuli]

It would be nice if Porcupine notified its users non-disruptively when a new version is out. For example, it could show a message in the status bar when it starts, or when the first tab is opened if no tabs open on startup (because of the user's settings or because they had no tabs opened when closing Porcupine).

Querying the latest version from GitHub's API is quite straight-forward:

>>> import requests
>>> requests.get('https://api.github.com/repos/Akuli/porcupine/releases/latest', headers={'Accept': 'application/vnd.github+json'}).json()['tag_name']
'v2022.07.31'

[rdbende]

It could also be on the welcome screen.

[Akuli]

One problem with using the welcome screen is that some people look at it very rarely: they use the setting to remember tabs when Porcupine is closed and opened again, and never close some tabs. So if we use the welcome screen, it would be good to also use automatically opened tabs.

[Moosems]

Why not just use the status bar?

[Akuli]

There is no status bar when you have no tabs open.

[Moosems]

Ahh. So then just use whichever one is open at start

[Moosems]

No tabs, there's the welcome page. Also, I'd love a global status bar for items related to the entire editor like reloading and so forth.