Add 4th row on first two layers for numbers / symbols

This adds a fourth row at the top on the letter / capital letter layers. On the letter layer, the row contains the numbers 0 through 9. On the capital letter layer, the row contains the symbols associated with the number keys.

This mimicks the behavior on password entries on Android and allows for easy access to common numbers / symbols without going deeper into the layers.

The 4th row is displayed conditionally if and only if the vertical space is sufficient to allow for keys with a height / width ratio of at least one. Otherwise, the previously existing three-row layout is used.

A few screenshots:

Closes: #116 (closed)