Previously grids would always take precedence when both a grid
and guide were within tolerance of a point.
Now, guides will always take precedence - since they have been
manually set by users we make the assumption that they have
been explicitly placed at highly desirable snapping locations,
and should be selected over the general grid.
Additionally, grid snapping was previously only done if BOTH
x and y could be snapped to the grid. We now snap to the nearest
grid line for x/y separately. This means if a point is close
to a vertical grid line but not a horizontal one it will still
snap to that nearby vertical grid line.
to all other pages
This allows resetting all other pages to use the guide configuration
for the current page. Since guides are now single page only (required
to handle mixed page size/orientation layouts), this is a shortcut
to allow guide configuration to be setup on a single page and then
easily transferred to all other pages in the layout.
Unlike in 2.x, Layouts in 3.0 adopt the standard UX of dragging
out rulers to create guide lines (instead of clicking on a ruler
position to create a guide)
The class is a QAbstractListModel subclass, making it easy
for creation of different views showing the guides in a layout.
Additionally, a QSortFilterProxyModel (QgsLayoutGuideProxyModel)
has been created to filter QgsLayoutGuideCollection to guides
with matching page numbers and orientations.
Tools can now indicate (via setFlags()) if they desire snapping
to be active for the tool. When a tool has the flag set,
any QgsLayoutViewMouseEvents it receives will have a snappedPoint()
available which returns the mouse event point snapped using
the layout's snapping settings.
