Improve upload dialogs for Project data

Several issues here:

:: We need a full modal dialog for this, blocking all other UI elements!
:: Controls directly associated to the dialog (submit, cancel) should go into the dialog window, not being offered as a global toolbar.
:: As uploads may take a long time, the user should be informed about this and that the upload process must not be interrupted.
:: While the upload is underway, a little processing animation could be displayed (probably just a spinning wheel, a progress bar might be too complicated).
:: Some users suggested enabling the ESC button as alternative to clicking on a "Cancel" button. Personally, I find that very unusual. This is a browser, not a desktop app.