Add a setting to add an offset to the bottom of the keyboard

[wittmane]

This adds the option to have extra padding at the bottom of the keyboard. This resolves #346.

I intentionally made this extra padding not clickable because the whole point is to shift the position of the keyboard, so I don't see a point in being able to click in this space, and depending on how large the offset is, registering clicks there seems like it would be unexpected for most users. We've had some debate on non-clickable spaces in the past, so if you disagree, I suppose I can change it.

I also only added support for portrait mode because I don't think there would be much point in landscape. In portrait mode, this offset can push the keyboard away from the navigation bar to avoid misclicks. Also, with phones having significantly smaller bezels than years ago, this can shift the keyboard back to a higher position that could be more comfortable, and due to the reduced bezels, there is extra vertical space to justify that. Landscape mode has much less available space, and doesn't have the same issue with the navigation bar being there, so I'm not sure why an offset would be beneficial there, but if you think there could be any value or want it simply for consistency, I could add it.