icon arrangement mega-patch for usability and maintainability

This splits the enum IconView::Flow into two enums
- IconView::Layout { Rows, Columns }
- IconView::Alignment { Left, Right }

Note that Qt::Alignment is not used to emphasize the fact that
these two properties are orthogonal (layoutDirection() is used
e.g. to place the scrollbar at the correct side of the screen,
whereas IconView::Alignment is about the side of the screen
the user want their icons to stick to.

This patch greatly improves the readability of the IconView
layouting code, as well as the user-friendliness of the
applet's Display configuration page, with a distinct separation
of the two layout-related factors.

Corresponding context menu actions have been added. They update
the configuration dialog combos when selected if the dialog is
open at that moment.

CCBUG:306537