KFileItemModelRolesUpdater: reset size and count before updating folders

KFileItemModelRolesUpdater: reset size and count before updating folders

Folder count would not update properly when user would delete file from
a folder, or add a new file to it.

Previously when size value is set to -2 after update, the update will
never be called again unless user presses F5. This change will instead
reset that -2 to 0 whenever we are requesting for calculating
directory sizes.

We never updated the count when a file was deleted, so that has been added as well.

This change also calculates the item counts from the processedAmount, which is the total amount of items we're processing. From there we remove the unwanted items and get the final count.

For remote files, we set the count to -1 since we don't calculate them.

BUG: 500502