common/file-helpers.c: share file_exists() to reduce duplication

src/common/file-helpers.c

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <sys/stat.h>
+#include "common/file-helpers.h"
+
+bool
+file_exists(const char *filename)
+{
+	struct stat st;
+	return (!stat(filename, &st));
+}

src/common/meson.build

@@ -2,6 +2,7 @@ labwc_sources += files(
   'buf.c',
   'dir.c',
   'fd_util.c',
+  'file-helpers.c',
   'font.c',
   'grab-file.c',
   'graphic-helpers.c',