No progress bar shown when parsing BinaryView headers

[CouleeApps]

When opening a binary, the header is parsed (BinaryView::Init) on the UI thread before a progress bar appears. In cases of large binaries, this can take tens of seconds during which the UI is unresponsive and no indication of progress is shown.

We should show the progress bar before parsing the headers, and include it in the progress percentage. Optionally, we should parse it on a background thread so the UI thread does not hang.

[plafosse]

This is kind of a duplicate of this: #540 though this issue offers a better explanation of whats going on. I'll close the other one in favor of this one.