datetime: implement compare method
functions: initial sort function
local-source: more verbose stub messages
manager: fixed indent issues
task: initialize date at construction time
list-row: row must own the list references

Otherwise it is dereferenced and destroyed.
task-row: initial task handling implementation
window: implemented task handling
window: change gear icon and add a popover menu
window: use GtkListBox::row-selected instead of GtkListBox::row-activated
schema: added priority field
task: add instance member 'priority'
local-source: translatable source name
new-task-dialog: format time spinbuttons' output
window: bind property instead of manually handle visibility
sort: use priority as first compare
new-task-dialog: refactor some code
lists-dialog: stub lists dialog implementation

A lot of work must be done.
local-source: implemented get_lists & create_list, fixed indent issues