src: typos and grammatical (English) cleanups.

[erictheise]

I've restricted my changes to comments, help, and error messages.

[erictheise]

@jean-emmanuel, there are a few occurrences of the world catched in the code and I wasn't positive whether you meant cached or caught but if you clarify I'd be happy to add the correct change to this PR.

[erictheise]

My IDE caught two other typos that are functional:

• x-positionned and y-positionned should have a single 'n'.
• fullscreen_unnavailable should also have a single 'n'.

Happy to change those in this PR as well, just let me know @jean-emmanuel.

[jean-emmanuel]

All occurrences of "catched" can be corrected to "caught".

Happy to change those in this PR as well, just let me know @jean-emmanuel.

Yes, please :)

[erictheise]

Good to go @jean-emmanuel.

[jean-emmanuel]

Thanks :)