add Categories

[Simon-Laux]

often you have subtasks like desktop: fix bug 123, desktop: release and desktop: review pr.

But in statistics you want to have a chart that lists time spend in categories, not time spend across subtasks.

Options for adding categories:
A. add category as seperate field on entry or drop down -> might be annoying?
B. implicitly: using a special syntax in label (category: label like "dev: desktop", "comunication: chat") <- my personal favourite
C. an special editor to match all existing label to a category -> also annoying and also the most coding work of the 3 options.

• allow specifying categories
• show statistics for categories (like a donut/pie diagram)