[ProgressBar] Linux implementation, currently only for Ubuntu using Unity api (+1 squashed commit)
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[4d33a69] [ProgressBar] fix window compile error (+1 squashed commits)

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[1738c18] [linux] placeholder for SetProgressBar (+1 squashed commit)
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[8dd41ec] [WIN] implement SetProgressBar for windows platform (+1 squashed commit)
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[79dca6e] [WIN] implement SetProgressBar for windows platform (+1 squashed commit)
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[1be7893] [MAC] implements SetProgressBar with value 0 - 1,  value > 1 means Indeterminate , value < 0 will turns of the progress bar. Currently still have problem with animating the Indeterminate state, and the progress bar color is fixed to blue
Merge pull request #2175 from jtg-gg/ProgressBar

[ProgressBar] OSX, Win, Ubuntu implementation