regedit : exporting keys

[Vincent Pelletier]

I'm curently working on #824 bug (\0 character is added to REG_MULTI_SZ 
registry values) and while hunting it I found that regedit uses it's own 
function to write to a file instead of using the RegSaveKey API.
Although it shouldn't solve the bug, it should be interesting to use the 
API.

I also got 2 bugs with the export function :
- the "save as" window got a square shape, not a rectangle (right side 
cuted, down one too downward)
- regedit always exports the whole hive, not only subkeys & values

Vincent Pelletier