Gui: added taskbar support

[rodlie]

• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

Enables rendering progress in the taskbar icon on Windows and macOS.

Show a few screenshots (if this is a visual change)

macOS:

Didn't have time to take a screenshot on Windows, it looks like any other app that uses the API (green background fill on the icon based on progress).

Have you tested your changes (if applicable)? If so, how?

• macOS 10.13
• Windows 10

Futher details of this pull request

See #169

[devernay]

thanks!

[devernay]

it may break the Qt5 build when merging into RB-2.5.
I'll do the check for macOS, but are you able to check building with Qt5 on Windows @rodlie

[rodlie]

but are you able to check building with Qt5 on Windows @rodlie

I got a new computer today for Natron development on Windows. Will setup latest MSYS2 (and MSVC, but will not use it yet).

Should be able to test against Qt5 on MSYS2 tomorrow (best case).