Make cmd_resize_tiling_direction work with pixels

Introduces resize_neighboring_cons in resize.c which is also used by
resize_graphical_handler.

Co-authored-by: Andrew Laucius <andrewla@gmail.com>
Authored original code and tests in #3240. I rewrote most of the
resizing code and fixed the failing tests.

include/resize.h

@@ -14,3 +14,19 @@
 bool resize_find_tiling_participants(Con **current, Con **other, direction_t direction, bool both_sides);
 
 void resize_graphical_handler(Con *first, Con *second, orientation_t orientation, const xcb_button_press_event_t *event);
+
+/**
+ * Resize the two given containers using the given amount of pixels or
+ * percentage points. One of the two needs to be 0. A positive amount means
+ * growing the first container while a negative means shrinking it.
+ * Returns false when the resize would result in one of the two containers
+ * having less than 1 pixel of size.
+ *
+ */
+bool resize_neighboring_cons(Con *first, Con *second, int px, int ppt);
+
+/**
+ * Calculate the given container's new percent given a change in pixels.
+ *
+ */
+double px_resize_to_percent(Con *con, int px_diff);