Add show-today icon

Used in KAlarm's date picker widget to go to today's date. The icons are currently installed as part of kalarm, but it would be better if they could be incorporated into the breeze package.

The icon is similar to that used by Thunderbird for its "today" control. See https://bugs.kde.org/show_bug.cgi?id=470451 for the rationale.

I'm not sure about the requirements for SVG files in the directories 16@2x/ etc, which I haven't provided in this merge request. I hope these can be added by people who know about such things.