Application: Implement `startup()`

[Marukesu]

Handy initialization and CSS loading is done here now, the new-window and quit action are implemented here too since there's no reason for them to be part of the window class. one behaviour diference is that the quit action now closes all windows instead of only the current window matching others elementary applications.

[jeremypw]

@Marukesu #709 is merged - a conflict needs to be resolved.

[Marukesu]

@jeremypw testing locally, the CI was falling because of the Idle in on_switch_page() in the MainWindow class. #720 should fix it.

[jeremypw]

For me it was failing in the "quit" action test. I tried to fix it but failed. We could always leave that test out and fix it later if necesssary.

[jeremypw]

720 now merged ... but CI still failing :disappointed: However, CI passed locally :man_shrugging:

[jeremypw]

Hmm, contrary to my previous diagnosis the CI is (now) failing on the "new-window" action test. See #723

[Marukesu]

Hmm, contrary to my previous diagnosis the CI is (now) failing on the "new-window" action test. See #723

the new-window test is fine, GLib.Test.skip() don't return by itself, so the assertion in the quit test got triggered.