Introduce custom style

This introduces a Qt vkbd custom style called "Breeze", which is based on the builtin style (https://github.com/qt/qtvirtualkeyboard/tree/dev/src/styles/builtin/default), with some tweaks that use Kirigami units, colors, and standard fonts (so it looks more like maliit). It also adds a basic sizing algorithm so that the keyboard is taller on narrow displays (ex. phones).