Improve accessibiity

Although swaync is already a great program, both in configurability and usability, I think it needs a little bit of improvement when it comes to accessibility.

I admit I haven't dug into the code to see if it works good with any screen reader or something like that, nor I have tried setting one up as I don't need them, but I think it could help a lot of people with visual impairments. My apologies if it's already "compliant" and I haven't noticed.

I don't know how much work it's needed, nor I'm fluent in Vala, but I could try to help somewhat when I get a hang of the language.

edit: my apologies, swaync is already navigable with the keyboard, but through shortcuts. I was trying to toggle the dnd button by getting to it with the arrow keys. Lovely that it already works!