Shotcut: v21.06.15 Release

This is a bug fix release to continue to address the problems found in version 21.05.01 after release, but there are a few low risk additions:

• Added Speed parameter to the Timer video filter.
• Added keyboard shortcuts to switch between the stock layouts: <kbd>Alt</kbd>+<kbd>1</kbd> through <kbd>6</kbd>.
• Added Import button to Playlist. (Thanks, @hbtalha!)
• Added an Apply button next to Properties > Speed to improve workflow when using mouse wheel.

New Bugs (in version 21.05.01/18)

• Fixed playback may hang when playing different Playlist items.
• Fixed alpha presets in Export.
• Fixed a possible crash when changing Settings > Preview Scaling.
• Fixed a possible crash when using Crop: Source and Size, Position & Rotate filters on the same clip.
• Fixed Size, Position & Rotate filters not reloading.
• Fixed reloading Keyframes for a filter on Output.
• Fixed changing Properties > Color Range for a clip in the Source player.
• Fixed playhead moves to the beginning of the Timeline when dragging an image from Source.
• Fixed a proxy may used instead of the original when dropping from file manager to Timeline.
• Fixed Fade In/Out on Timeline may add filters in wrong order.
• Fixed a crash trying to make thumbnail for an invalid playlist item when loading a project.
• Fixed Playlist > Play After Open not working.

Old Bugs

• Fixed Center bias in Crop: Source filter.
• Fixed a crash using Mask: From File and Size, Position & Rotate filters on the same color clip.
• Fixed the Export > H.264 High Profile preset was producing main profile on NVIDIA.
• Fixed a crash when using Paste after a File > Close or File > New.
• Fixed a Project name with a slash is accepted but fails.
• Fixed a possible crash when changing Settings > Theme.
• Fixed Mask: Simple Shape moves when deleting all keyframes: no longer allowed to delete the last keyframe; use the keyframe toggle button in the parameters area of Filters.
• Fixed maintaining the image center when reset Size, Position & Rotate > Zoom.
• Improved using the mouse wheel to zoom in Size, Position & Rotate filter.
• Improved the range of the new Zoom keyframe values when keyframes are changed or removed.
• Fixed custom Export preset may load with incorrect frame rate.
• Fixed Playlist > Replace may replaced with timeline and appear as <tractor> and cause a crash if the Project player it active.
• Fixed high CPU usage and a long wait after multiple Stabilize analysis jobs.
• Fixed inconsistent handling of keyframes when splitting a trimming a clip.