ProgressBar: Fix updating
ProgressBar: Fix updating

'updateCV.notify_one()' does nothing if the update thread is not
waiting for updateCV (in particular this happens when it is sleeping
on quitCV). So also set a variable to ensure that the update isn't
lost.

pikajude/nix,pikajude/nix,rycee/nix,rycee/nix,ehmry/nix,dtzWill/nix,pikajude/nix,ehmry/nix,dtzWill/nix,ehmry/nix,NixOS/nix,rycee/nix,rycee/nix,dtzWill/nix,ehmry/nix,NixOS/nix,NixOS/nix,ehmry/nix,dtzWill/nix,ehmry/nix,rycee/nix,NixOS/nix,dtzWill/nix,NixOS/nix