v0.5.4

this release changes:

• A new app icon and symbolic! (based on @bertob's design and with his help. Thanks a ton!)
• long awaited: write and edit text with the typewriter
• pdf import dialog with the added option to import aligned with the document pages ( one pdf page per document page )
• import bitmap images (e.g. screenshots) from the clipboard (thanks @RickStanley!)
• proportial shapes / input constraints (thanks @sei0o!)
• additional selector modes: apiece, intersecting path
• improved workspace browser, added sidebar to select between different workspaces (inspired by the `Paper` app)
• mark underneath other strokes with the 'Marker' brush style
• bug fixes