gio: Fix various implicit conversions from size_t to smaller types

Basically various trivial instances of the following MSVC compiler
warning:
```
../gio/gio-tool-set.c(50): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
```

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

gio/gfilenamecompleter.c

@@ -345,7 +345,7 @@ init_completion (GFilenameCompleter *completer,
   GFile *file, *parent;
   char *basename;
   char *t;
-  int len;
+  size_t len;
 
   *basename_out = NULL;