Indeterminate progress bar

[gan74]

I added an option to have the progress bar display an animated indeterminate loading status in the spirit of #1901.
It works by passing a negative fraction, varying the value moves the bar (so it's easy to vary the speed of the animation).

ImGui::ProgressBar(ImGui::GetTime() * -0.25f);:

ImGui::ProgressBar(ImGui::GetTime() * -0.2f, ImVec2(0, 0), "Loading...");:

_{^{forgive the bad looping}}

I am not sure if modifying the existing ProgressBar implementation is the right thing to do, or if adding a new function would be preferable.

[ocornut]

Thanks for your PR. This is now merged as c895e98, with minor tweaks and update for latest (as we recently added support for NaN, #7451).
The horizontal centering of overlay was wrong because of added ItemSpacing.
Thanks!