[build]: C and GTK 4.6.9 (Ubuntu 22.04)
[feat]: sqlite database backend
[feat]: backup and restore database (csv file)
[feat]: delete all menu item
[feat]: read out upcoming events for next seven days
[feat]: public holiday calendar marks
[feat]: calendar today and event colours
[feat]: basic scaling
[feat]: binary for 64-bit GTK4 distributions
[docs]: updated readme documentation