Download speed incorrect when resuming downloads

[JAJames]

When resuming a download, files which were already downloaded are accounted for as part of the download phase.

Reproduction steps:

1. Start installing the game
2. Cancel game installation
3. Restart the launcher
4. Start installing the game

Expected behavior: Already downloaded files do not count towards download progress and total
Actual behavior: Already downloaded files count towards download progress and total, causing incorrect download speed calculations.

[AlienXAXS]

Just as a note here:
Downloads will resume from where they left off already if the patch\download folder contains items, i am however unsure if the launcher does any hash checking on this which will be needed.

ToDo: Check if the already existing items are subject to hashing checks, if not implement this.

edit
Confirmed: Already existing files are hash checked.

[AlienXAXS]

This works with #22 when the user cancels the download and starts it again later - however an extra dialog perhaps upon pressing Cancel to state "Are you sure?" and "You can resume this download later" might be useful to the end user.

[JAJames]

With those changes in place, I'll rephrase this to focus on resolving the bug where resumed download files are checked and accounted for in the download phase rather than the verification phase.