Show archive reading status on the welcome screen

As suggested in bug 441529, this merge request replaces the progress bar in the status bar with progress bar on the welcome screen. (When the welcome screen is not visible, the progress bar in status bar is used instead. That is the same behavior as before this MR.)

The original approach shows only static text on the welcome screen (“Welcome to Ark”). That could be misleading when the archive opening was in progress because it provided buttons for opening and creating archives while loading.

Now, when the archive is being opened (or some other KJob-based job is in progress), the Ark shows the progress and description of the job on the welcome screen instead of the buttons and “Welcome to Ark”.

Previous welcome screen

The file name, after.png, relates to my previous merge request (!47 (merged)) that added the welcome screen. Ths is state before the change, not after.

Current welcome screen

The screen looks exactly the same as the previous one but, when archive is being opened, shows the progress bar: