Fix uploading sending same req# twice, fix #1009
- the `requestInProgress` flag is set during Ajax requests
- the flat is reset at the end of `handleResponseDom()`
- uploads are not done with an Ajax request (but a iframe form POST)
- the server responds to the POST with an XML embedded in HTML
- we use the same `handleResponseDom()` to process that XML
- this means that at the end, we incorrectly reset `requestInProgress`
- this has no impact, except if there an Ajax request is running
Fully reinit wPaint and show image in readonly
Minor: typo
Minor: styling of upload control
Don't show canvas warning when readonly