Implement focus_follows_mouse

Also contains two other small changes:

- Clicking any button will focus the container clicked (not just left)
- Remove seamless_mouse (doesn't make sense on wlroots)

sway/commands/focus_follows_mouse.c

@@ -0,0 +1,12 @@
+#include <string.h>
+#include <strings.h>
+#include "sway/commands.h"
+
+struct cmd_results *cmd_focus_follows_mouse(int argc, char **argv) {
+	struct cmd_results *error = NULL;
+	if ((error = checkarg(argc, "focus_follows_mouse", EXPECTED_EQUAL_TO, 1))) {
+		return error;
+	}
+	config->focus_follows_mouse = !strcasecmp(argv[0], "yes");
+	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
+}