Implement the midnight delay option

Closes #694.

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/preferences/Preferences.kt

@@ -183,6 +183,13 @@ open class Preferences(private val storage: Storage) {
             for (l in listeners) l.onCheckmarkSequenceChanged()
         }
 
+    open var isMidnightDelayEnabled: Boolean
+        get() = storage.getBoolean("pref_midnight_delay", false)
+        set(enabled) {
+            storage.putBoolean("pref_midnight_delay", enabled)
+            for (l in listeners) l.onCheckmarkSequenceChanged()
+        }
+
     fun updateLastHint(number: Int, timestamp: Timestamp) {
         storage.putInt("last_hint_number", number)
         storage.putLong("last_hint_timestamp", timestamp.unixTime)

uhabits-core/src/jvmTest/java/org/isoron/uhabits/core/preferences/PreferencesTest.kt

@@ -167,4 +167,12 @@ class PreferencesTest : BaseUnitTest() {
         assertTrue(prefs.showArchived)
         assertFalse(prefs.showCompleted)
     }
+
+    @Test
+    @Throws(Exception::class)
+    fun testMidnightDelay() {
+        assertFalse(prefs.isMidnightDelayEnabled)
+        prefs.isMidnightDelayEnabled = true
+        assertTrue(prefs.isMidnightDelayEnabled)
+    }
 }