Add tiny button to jump to today

[unode]

After navigating across months/years it would be convenient to have a tiny button to return to today. There are already key shortcuts but having this for mouse navigation would be useful.

Taking some inspiration from Lightning in Thunderbird:

What I'm referring to is the little circle.

[m-bartlett]

For what it's worth, there are keyboard shortcuts to accomplish this, specifically g and home:

gsimplecal/src/MainWindow.cpp

Lines 160 to 161 in 55adc41

	{GDK_KEY_g, 0, goTodayCallback},
	{GDK_KEY_Home, 0, goTodayCallback}};