applets/systemtray: add adjustable spacing, and change it in Tablet Mode

Over the years we've changed icon spacing many times. And every time we
do, we get bug reports about it from people who were happier with the
previous value. Typically we close these bug reports as RESOLVED
INTENTIONAL, but it always leaves a sour taste in the mouth. People's
preferences for spacings and paddings are legitimate.

In addition, a larger spacing can be quite useful in Tablet Mode, to
increase the size of the touch targets to be closer to the size of the
user's finger.

This commit adds a simple UI to switch the spacings of the visible items
between three values, with the middle one that matches the current
spacing being the default. While Tablet Mode is activated, the spacing is
automatically set to its highest value and made non-user-adjustable,
just like we already do this for the icon size setting.