Add missing event-handling functions

[Kevinpgalligan]

There are now event-handling functions for clicking and hovering, so that users don't have to use the SDL2 methods directly. We're still missing such functions for text input and for click release.

Also need to update the documentation to use these new functions, as it currently refers to the SDL2 methods.

[Kevinpgalligan]

Just to note: these functions have been added, but they're not entirely consistent with the documentation and we still want to agree on a final interface.

[Kevinpgalligan]

See: #156

[Kevinpgalligan]

Done: #156

Keeping this open for:

• Mousewheel.
• Controller stuff.
• Any other SDL2 input types we haven't covered.
• Window resize events.
• Window close events.
• Etc.