Bugfix/improved quit handling

[Blizzardsev]

Resolves the following:

• Issue where consequences for force quitting could be avoided by repeatedly selecting the close option on the game window
• Issue where force quitting and backing out repeatedly could result in the game restarting from the greeting, due to Ren'Py falling out of context
• Patch some function names to match newer conventions
• Fix some incorrect documentation

Closes #850

[Blizzardsev]

As a heads up, testing for this will require a range of scenarios to make sure all logic is functioning as we expect:

• A normal goodbye should result in Natsuki welcoming the player back as usual
• Force quitting, then selecting Nevermind, then saying goodbye should result in Natsuki welcoming the player back as usual
• Force quitting, then selecting ... should result in the game closing and Natsuki being annoyed when the player returns
• Force quitting by clicking the close option repeatedly should result in the game closing and Natsuki being annoyed when the player returns
• Attempting to force quit by clicking the close button once while in a menu/game should result in the game remaining open, and Natsuki welcoming the player back as usual when saying goodbye later as usual

Any other scenarios should be tested if they come to light.

[Blizzardsev]

Merging this in; pretty confident all cases have been covered by this point. We'll re-run these when release nears.