Add default icons for the "Recent Sessions" and "All Sessions" menus

There are two benefits:

1. All items in the Sessions menu have icons, which looks neat.
2. When you add one of these actions to the toolbar, they have icons by default, which looks better.

Here's how it'll look now:

To make it possible, KateSessionsAction is given the capacity to accept a QIcon as its first argument, and all uses of it are ported to do so.

This is an upstreaming of a local change I made ages ago and forgot about, so I thought I'd contribute it.