* feat: apply margin to `osd-margin-y` and `osd-margin-x`
The margins were already set for `osc-margins` and
`user-data/osc/margins`, but osd messages have their own properties
`osd-margin-x` and `osd-margin-y`.
Offsetting `osd-margin-x` and `osd-margin-y` by our margin prevents
overlap of osd messages with the GUI.
I don't like this solution, but anything cleaner would probably require converting chapter indicators into individual elements and than managing/updating their positions and sizes in relation to growing timeline, which seems like a lot more work and code, so this'll have to suffice.
closes#370
