Adaptive width

The goal of these changes is to solve the issue described in #38 (closed).

When the width of the window is 700 pixels or smaller, it will split the bottom controls into two rows:

When the width of the window is greater than 700 pixels, it will revert back to the current layout:

(The screenshot above is rotated from a VM based on the Librem 5 Linux phone, hence the wonky display).