gui: Implement descriptive tree items.

gui: Implement descriptive tree items.

A restriction of the current gui module is that the text shown in the
menu items must also be a valid identifier for the corresponding bash
function. In particular no whitespace is allowed there.

To overcome this restriction we introduce the concept of a menu item
identifier and an optional menu item description. If the description
is absent, the identifier is used as the description, providing
backward compatibility.