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
Prevent the rendering progress dialog window from being resizable

This was the cause for the big empty space between the progress bar and
the action area: GTK3's size calculations were based on the height of the window
before its width would be resized.

Somehow, disabling window resizing works around this.

With the new text paragraph (or, alternatively, the default width of 400 px),
the dialog doesn't really need to be resized by users anyway.
render: Update the time and filesize estimates independently from percentage

Instead of doing it all in one go everytime the pipeline changes position,
start independent estimate timers once we have enough data to make a good guess.

The progressbar remains fluid while other estimates fluctuate much less,
significantly improving perceived accuracy of those estimates.