Allow values to be changed by sliding

[thykka]

The ++ and -- buttons exist only to allow changing values with less taps. It's better than nothing, but still quite fiddly.

Instead, we could allow the user to slide their finger over the input area to make coarse changes. The sliding action should probably act as relative to the current value, rather than an absolute one.

If this input method works well, we might be able to increase the precision of + and - buttons too.