AgendaView: fix misaligned layout of header & footer columns

The old code relied QHBoxLayout equally distributing the content across the header and footer. Though that will try to be more graceful to elements hinting to like bigger sizes, thus then screwing over alignment with the columns in the actual agenda.

The new code instead uses a custom QLayout subclass which enforces equal header columns.

The patch also fixes the bad placement of week decoration elements messing into the actual day headers, and places it aligned with the time labels, where there is space.

Examples:

Tiny window (poor-human testing of RTL with --reverse):

Giving a bit more space:

Fullscreen (on small laptopscreen, so needed to switch to January to have long labels ;) ):

@gditchfield @winterz