Add default key-binding for DRAG_RELATIVE (#117)

Ctrl-Button1 now has a relative drag using the XC_fleur cursor.
XC_fleur is normally the cursor for "size all" action, which has 4
arrows pointing to 4 directions.

Co-authored-by: NRK <nrk@disroot.org>

window.c

@@ -51,7 +51,7 @@ static struct {
 	int name;
 	Cursor icon;
 } cursors[CURSOR_COUNT] = {
-	{ XC_left_ptr }, { XC_dotbox }, { XC_watch },
+	{ XC_left_ptr }, { XC_dotbox }, { XC_fleur }, { XC_watch },
 	{ XC_sb_left_arrow }, { XC_sb_right_arrow }
 };