Add custom timers

[dar5hak]

Closes #6.

• [x] Add a way to actually change interval of a Reminder object. (Changing interval property isn't enough.)
• [x] Add GSettings to remember custom timers
• [x] Provide a UI
• [x] Test
• [x] Update translations
• [x] Update screenshot and (potentially) readme
• [x] Add margins
• [x] Display number on slider

[dar5hak]

@NathanBnm @welaq @siigdev With this pull request, the UI now looks like this:

I will need your help translating those durations and that subheading. If you have time, could you add these translations so that when I release a new version, non-English-speaking users don't have to look at English strings?

Also, as you can see, I have added colons after all 5 labels (Eyes:), since that's the convention for slider labels.

I replicated the same in each language, but I don't know if that's the correct convention outside of English. Is my assumption correct?

[NathanBnm]

@dar5hak that looks great! Yes you're right, there isn't the same formatting in all languages. I'm going to check that.