FactController: Fix start date picker modifying date twice

Previously, setting the `date` property would (in addition to setting
the property itself and the `default_day` property of the dayline) also
update the fact's start and end, moving those by the same delta as the
date property was modified.

However, this caused a problem when changing the start date using the
date picker, since `on_start_date_changed()` would already update the
fact's start and end and then also update the `date` property, which
would then update the fact *again*, effectively applying the delta
twice.

This commit fixes this problem by changing the `date` setter to no
longer update the fact. This means that the `date` property now only
tracks the currently selected day of the dayline (and the UI as a
whole), making the setter and getter a bit better matched.

This also means that any changes to the fact's start and end must be
done separately from modifying the `date` property. There are two places
that currently update the `date` property:
 1. The `on_start_date_changed()` method, which already updates the fact.
 2. The `increment_date()`, which is changed by this commit to update
    the fact.

Note that `increment_date()` now uses the `Fact.date` property setter to
modify the fact, which mostly implements the same logic as the code that
it replaces, except there is a small change in behavior when
`fact.start` is `None`.  However, that is a corner case that probably
almost never occurs and has other problems, so this is left for a later
refactor.

This fixes projecthamster#590.

src/hamster/edit_activity.py

@@ -134,15 +134,8 @@ def date(self):
 
     @date.setter
     def date(self, value):
-        delta = value - self._date if self._date else None
         self._date = value
         self.cmdline.default_day = value
-        if self.fact and delta:
-            if self.fact.start_time:
-                self.fact.start_time += delta
-            if self.fact.end_time:
-                self.fact.end_time += delta
-            # self.update_fields() here would enter an infinite loop
 
     def on_prev_day_clicked(self, button):
         self.increment_date(-1)
@@ -161,6 +154,7 @@ def get_widget(self, name):
     def increment_date(self, days):
         delta = dt.timedelta(days=days)
         self.date += delta
+        self.fact.date = self.date
         self.update_fields()
 
     def show(self):