This removes the custom label-based triangles and replaces them with proper ToolButtons
using proper icons. It also adds a "Today" button to return to the current day.
The tooltips on the buttons reflect the actual action ("Previous Month", "Previous Year",
"Previous Decade", depending on the zoom level).
Furthermore, the day names and week numbers now use the regular font size but they're still
visually disconnected. The day names row is hidden in the zoom levels now.
REVIEW: 126960
BUG: 336124
BUG: 348362
BUG: 358536
4ebf130ee6