Callipeg: 2D Animation App

Fixes: - Exported sound volume: exported projects now follow the clip, layer and master volume sliders (and hidden audio layers stay silent) instead of always exporting at 100%. - Audio import: audio files imported through the timeline's file picker no longer play as noise or drift out of sync. A project whose audio conversion failed earlier is repaired when it opens. - Image-sequence import: importing a folder of images works again. - Damaged compressed projects: a compressed project with a damaged archive now opens with everything that could be recovered and tells you which files were lost, instead of not opening or retrying endlessly. - Missing brushes after updating: PencilHB and Round Opacity brushes, and brushes made from them, are back after updating - Brush texture: textured brushes place their texture at a new random position on every stroke again, instead of the same spot each time. - Brush thumbnails: selecting a brush no longer changes its thumbnail, and textured brushes show their texture in the list even when they are not selected. - Export panel: no longer freezes the app on large projects when it opens, when you move the frame slider or when you toggle the background. Exporting from the gallery now opens projects in the background. - Splitting sheets: a split that keeps its content no longer leaves the new sheet blank or partly drawn, even after repeated splits and undos, and a split size of 0 no longer freezes the app. - Layer colour: the timeline's layer colour button updates as soon as you pick a new colour. Stability: - Fixed a memory leak on every brush switch, thumbnail render and brush-size change that could get the app closed during long sessions. Switching brushes also does less work - Fixed a crash when exporting several projects from the gallery at once.