Treeshaking and datepickers

Real good stuff

• Datepickers: With input and standalone.
• Tree shaking (allows to import only components you need, reducing bundle size): should work with vue-cli and vite (ssr) unless we messed something up :X.

Internal changes and smaller stuff

• We're gradually moving from mixins to composition API. That gives us good typing; also composables are easy to expand. Downside is the steep(er) learning curve :/.
• Docs tweaks (anchors, file structure, markdown tweaks).
• Simplify translations handling.
• Small fixes.