Bug 876598 - Remove dead space on side and bottom of keyboard

Fix for [#876598](https://bugzilla.mozilla.org/show_bug.cgi?id=876598). This PR removes the padding-bottom of the keyboard area and makes the last keyboard row higher instead. It also enlarges the key container for the first and last key in a row when the full width is not being used, to fill up the row completely. This reduces mistapping in the keyboard as there is no more dead space.