New feature: adjust gaps with floating_mod+scroll

I made this configurable but I didn't make the command for it. That's
left as an exercise to an eager contributor.

    mod_scroll_behavior [gaps inner|gaps outer]

Would merge implementions of more behaviors for mod+scroll, if anyone
has some neato ideas.

include/config.h

@@ -163,6 +163,13 @@ enum edge_border_types {
 	E_BOTH		/**< hide vertical and horizontal edge borders */
 };
 
+enum floating_scroll_behavior {
+    FSB_GAPS_OUTER,     /**< Adjust outer gaps */
+    FSB_GAPS_INNER      /**< Adjust inner gaps */
+    // Note: in the future I expect to see more things you can do with the scroll
+    // wheel than maniuplating gaps
+};
+
 /**
  * The configuration struct. The result of loading a config file.
  */
@@ -181,6 +188,7 @@ struct sway_config {
 	uint32_t floating_mod;
 	uint32_t dragging_key;
 	uint32_t resizing_key;
+    enum floating_scroll_behavior floating_scroll; // TODO: command to set this
 	enum swayc_layouts default_orientation;
 	enum swayc_layouts default_layout;
 	char *font;

sway/config.c

@@ -159,6 +159,7 @@ static void config_defaults(struct sway_config *config) {
 	config->floating_mod = 0;
 	config->dragging_key = M_LEFT_CLICK;
 	config->resizing_key = M_RIGHT_CLICK;
+	config->floating_scroll = FSB_GAPS_INNER;
 	config->default_layout = L_NONE;
 	config->default_orientation = L_NONE;
 	config->font = strdup("monospace 10");

sway/handlers.c

@@ -647,6 +647,31 @@ static bool handle_pointer_button(wlc_handle view, uint32_t time, const struct w
 	case M_SCROLL_DOWN:
 		break;
 	}
+	if (!(modifiers->mods ^ config->floating_mod) &&
+			(button == M_SCROLL_UP || button == M_SCROLL_DOWN)) {
+		switch (config->floating_scroll) {
+		case FSB_GAPS_INNER:
+		case FSB_GAPS_OUTER:
+			{
+				int amount = button == M_SCROLL_UP ? -1 : 1;
+				int i,j;
+				for (i = 0; i < root_container.children->length; ++i) {
+					swayc_t *op = root_container.children->items[i];
+					for (j = 0; j < op->children->length; ++j) {
+						swayc_t *ws = op->children->items[j];
+						// TODO: adjust outer gaps ws->gaps = 0;
+						if (config->floating_scroll == FSB_GAPS_INNER) {
+							container_map(ws, add_gaps, &amount);
+						} else {
+							ws->gaps += amount;
+						}
+					}
+				}
+				arrange_windows(&root_container, -1, -1);
+				break;
+			}
+		}
+	}
 
 	// get focused window and check if to change focus on mouse click
 	swayc_t *focused = get_focused_container(&root_container);