Implement new feed and subscriptions groups

- Introduce Groupie for easier lists implementations
- Use some of the new components of the Android Architecture libraries
- Add a bunch of icons for groups, using vectors, which still is
compatible with older APIs through the compatibility layer

app/src/test/java/org/schabi/newpipe/local/subscription/FeedGroupIconTest.kt

@@ -0,0 +1,30 @@
+package org.schabi.newpipe.local.subscription
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class FeedGroupIconTest {
+    @Test
+    fun `No gaps and repeated ids`() {
+        val usedIds = HashSet<Int>()
+
+        for ((shouldBeId, currentIcon) in FeedGroupIcon.values().withIndex()) {
+            val added = usedIds.add(currentIcon.id)
+            assertTrue("Repeated ids (current item: ${currentIcon.name} - ${currentIcon.id})", added)
+
+            assertEquals("Gap between ids detected (current item: ${currentIcon.name} - ${currentIcon.id} → should be: $shouldBeId)",
+                    shouldBeId, currentIcon.id)
+        }
+    }
+
+    @Test
+    fun `No icons pointing to the same attr`() {
+        val usedIcons = HashSet<Int>()
+
+        for (groupIcon in FeedGroupIcon.values()) {
+            val added = usedIcons.add(groupIcon.drawableResourceAttr)
+            assertTrue("Repeated icon (current item: ${groupIcon.name} - ${groupIcon.id})", added)
+        }
+    }
+}

app/src/test/java/org/schabi/newpipe/local/subscription/services/ImportExportJsonHelperTest.java

@@ -3,7 +3,6 @@ package org.schabi.newpipe.local.subscription.services;
 import org.junit.Test;
 import org.schabi.newpipe.extractor.subscription.SubscriptionExtractor;
 import org.schabi.newpipe.extractor.subscription.SubscriptionItem;
-import org.schabi.newpipe.local.subscription.ImportExportJsonHelper;
 
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;