Implement per-tag primary_count and primary_ratio

By default, each tag mask will use the default count and ratio. If the
mask gets modified by any of the commands, it gets added to a hash map.
When changing tag masks, the current count and ratio are stored, and
they're used again later if you switch back to that mask.

This commit also adds primary_count and primary_ratio to the general
settings for the config, so users can set a default count/ratio to use.

examples/config.kdl

@@ -1,5 +1,9 @@
 // Whether new windows should go to the top or bottom of the window stack
 attach_mode top
+// Number of windows in the primary stack
+primary_count 1
+// Proportion of output width taken by the primary stack
+primary_ratio 0.55
 // Whether mousing over a new window should move focus
 focus_follows_pointer #true
 // Whether the focus should warp to the center of newly-focused windows