There are some user complaints that Imager hangs at "writing 0%"
Mention in the progress status what it is actually doing prior
to writing (e.g. opening storage, blkdiscarding,
zero'ing out MBR, starting download) to help diagnose
potential issues.
We originally used libcurl for both downloading images from
Internet and reading local files to have the same code path for both.
It doesn't work that well in practice, as Qt and libcurl are not
on the same page how special characters such as Chinese characters
are represented in a local file URL.
So create a new class to handle local files.
Closes#76