Kalendar/UI: Improve usability

This MR improves the usability of the kalendar in following ways:

1. When adding task, I could not see new tasks being added. This is
   because tasks are not standalone and they are tied to some calednar.
   This is not obvious for first time users and this is now
   communicated clearly.

2. When clicking the + button in the bottom right corner, it doesn't
   actually add any task though the  tooltip says the button adds task.
   I did not notice the text input first and thought I would get some
   dialog to enter the task after clicking the button. This is now handled
   correctly and user is let to know what went wrong instead of button
   doing nothing.