Update to 2020.09.2

Changelog:

• New Features
  • A plugin system allows extending the Pitivi functionality medium-term, targeted for teams of editors.
  • A developer console plugin allows interacting with the project in a Python console.
  • Custom UI replaces the automatically generated UI for a handful of effects.
  • Easy Ken-Burns effect.
  • The new Greeter perspective replaces the Welcome wizard dialog and allows a slick selection of a recently opened project.
  • When being resized, the Viewer shows the percent of the actual widget size in relation to the project video size.
  • The Viewer size snaps at 50% when being resized.
  • Scaled proxies if optimized media is too much for your machine.
  • Timeline markers.
  • Support for nested timelines by importing entire XGES files as a clip.
  • The Effects Library has been redesigned to allow quick access to effects.
  • Ability to favorite effects in the Effects Library.
  • Improved workflow for adding effects.
  • The refreshed clip effects UI allows working on multiple effects at a time.
  • Refactored Media Library using same logic for different view modes.
  • Refactored Render Dialog UI to avoid overwhelming people. Tell us what you think about it.
  • Restoring the editing state when reopening a project.
  • Safe areas visualization in the Viewer.
  • Easy alignment for video clips.
  • Composition guidelines in the Viewer.
  • Solid color clips.
  • Ability to mute or hide an entire layer.
  • Interactive intro so newcomers get familiar with the names of the UI elements.
  • The action search is a shortcut to everything possible in the timeline, if you can find it. Press "/".
  • New keyboard shortcuts for pros.