build: remove autogen warnings

Copied autogen.sh from folks projects where pwithnall cares for
autotools stuff.
date-selector: make it GtkMenuButton descendant

Added GtkBuilder's UI file definition
window: use edit-dialog on new event action

New event action triggered from the menu on the header bar or the app
menu should not use new-event popover.
search-view: remove unused GtkSizeGroup
date-selector: fix memory leak
utils: fix memory leak
month-view: fix memory leak

Remove unneeded debug instructions
view: remove ::mark_current_unit

New event action will open edit-dialog directly so this method is not needed