workaround for icons not rendered for apps existing before waybar launch

[RobertMueller2]

This is a workaround for #3366 .

Using gdb, I can see that handle_app_id can sometimes run when tbar_->icon_themes() has 0 elements. This happens in multi output settings and can result in empty icons for apps that have been started before waybar.

It looks to me that the Tasks can be present before Taskbar::Taskbar has run, I could also confirm this by temporarily setting a bool flag in the constructor. I would appreciate any more detailed explanation why that is even possible, because I don't understand it at all.

With an explanation there's probably better ways to do this. :) But the code in this PR addresses this by setting the app_ids for all of the taskbar's tasks again at the end of Taskbar's constructor, leading to another icon lookup with icon_themes() available for handle_app_id.