[WIP UNTESTED] render: Provide an estimation of the output file size

Fixes bug #637079
[SQUASH] render: Provide an estimation of the output file size

Fixes bug #637079
Redesign the rendering progress dialog

- Replace the header by a small informational text on what affects render perfs
- Make the window title serve as the new dialog header
- Remove the encoding FPS and multipass labels, they're geeky and mostly useless
[FIXUP] better precision!