Fix modifying file type associations under MSW as non-admin

The existing code tried writing to HKEY_CLASSES_ROOT which doesn't work, in
general, for normal, i.e. non administrator, users, under any post-XP versions
of Windows.

Fix it to write to HKEY_CURRENT_USER\Software\Classes, which is the part of
HKEY_CLASSES_ROOT for the current user and to which we do have write access,
and only continue to use HKEY_CLASSES_ROOT itself for reading.

(this is a backport of 0ed580f451 from master)

docs/changes.txt

@@ -634,6 +634,7 @@ wxMSW:
 - Fix RegisterHotKey() with negative IDs (troelsk).
 - Fix event object type for wxEVT_SPINCTRL events.
 - Fix bug if wxToolBar tool was deleted from its own handler (Artur Wieczorek).
+- Fix creating or modifying file associations under recent Windows versions.
 
 wxOSX: