Improve progress bar handling for backup / restore / image flash

The progress bar will now be updated during image backups, restores
and during image flashing (except for sparse images which will require
significant changes to libsparse, and except for mtd nand using
flash_utils).

The progress bar will now be updated mid-file for file systems (tar) so
the user will see changes even during large file backup / restore.

Add a new progress tracking class to simplify handling of progress bar
updates. The class will only update the progress bar 5 times a second to
reduce the CPU load from updating the GUI frequently which does affect
backup times.

Change-Id: Iff382faef3df1f86604af336c1a8ce8993cd12c5