Implement floating_modifier and mouse operations for floating views

This implements the following:

* `floating_modifier` configuration directive
* Drag a floating window by its title bar
* Hold mod + drag a floating window from anywhere
* Resize a floating view by dragging the border
* Resize a floating view by holding mod and right clicking anywhere on
the view
* Resize a floating view and keep aspect ratio by holding shift while
resizing using either method
* Mouse cursor turns into resize when hovering floating border or corner

sway/commands/floating_modifier.c

@@ -0,0 +1,30 @@
+#ifdef __linux__
+#include <linux/input-event-codes.h>
+#elif __FreeBSD__
+#include <dev/evdev/input-event-codes.h>
+#endif
+#include <xkbcommon/xkbcommon.h>
+#include <xkbcommon/xkbcommon-names.h>
+#include <strings.h>
+#include "sway/commands.h"
+#include "sway/config.h"
+#include "list.h"
+#include "log.h"
+#include "util.h"
+
+struct cmd_results *cmd_floating_modifier(int argc, char **argv) {
+	struct cmd_results *error = NULL;
+	if ((error = checkarg(argc, "floating_modifier", EXPECTED_EQUAL_TO, 1))) {
+		return error;
+	}
+
+	uint32_t mod = get_modifier_mask_by_name(argv[0]);
+	if (!mod) {
+		return cmd_results_new(CMD_INVALID, "floating_modifier",
+				"Invalid modifier");
+	}
+
+	config->floating_mod = mod;
+
+	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
+}