Group layout, focus, proxy window logic into one, and add command line entry point

[felixfung]

Task idea comes from discussion in #70. We have the following use cases:

preference	alt-tab	expose	paging
priority	windows z-order	window position	virtual desktop location
prev/next sorting	windows z-order	expanded layout position (per column)	per virtual desktop (per row)
position preservation	depends on user preference	preserve relative positioning loosely but not overlapping	preserve within virtual desktop, expand virtual desktop positioning
animation	not important	important	so-so
virtual desktop filtering	no preferences	prefers single desktop	all desktops

Given that, the windows layout, focus logic, proxy window (only for paging, where a proxy window represents the virtual desktop) are distinct for each of the three use case, and that a user may want to invoke each use case with different user settings of their choice, we would want to group the windows layout, focus logic and proxy window logic into one code path, and then add command line entry point to select/toggle each of the three.

[felixfung]

Some hard-coded behaviour:

behaviour	alt-tab	expose	paging
layout	always XD	user config	always grid
prev/next ordering	always row-row	always column-column	always row-row
animation	never animation	user config	user config
virtual desktop filtering	user config	user config	all desktops