Add weight for TaxTypes

This needs tests. Let's not define default weights on the existing plugins, there is no need. The shipping plugin can define its own weight that's higher than 0.

+  /**
+   * Helper function for sorting Tax Types by weight.
+   *
+   * @param \Drupal\commerce_tax\Entity\TaxTypeInterface $a
+   *   Tax Type A for comparison.
+   * @param \Drupal\commerce_tax\Entity\TaxTypeInterface $b
+   *   Tax Type B for comparison.
+   *
+   * @return int
+   *   Returns -1 if A<B; 1 if A>B; 0 id A=B
+   */
+  protected function sortTaxTypes(TaxTypeInterface $a, TaxTypeInterface $b) {
+    $aConfig = $a->getPlugin()->getPluginDefinition();
+    $bConfig = $b->getPlugin()->getPluginDefinition();
+    if ($aConfig['weight'] < $bConfig['weight']) {
+      return -1;
+    }
+    elseif ($aConfig['weight'] > $bConfig['weight']) {
+      return 1;
+    }
+    else {
+      return 0;
+    }
+  }
+

Every config entity already has a sort() method, we should override it on the TaxType class, and call it from there.
We also don't use camel case in code.

Here's an initial cleanup. Still needs tests.

       $this->taxTypes = $tax_type_storage->loadByProperties(['status' => TRUE]);
+      uasort($entities, [TaxType::class, 'sort']);

$entities should be $this->taxTypes.

Included an updated patch with the switch to $this->taxTypes as pointed out in #7. Also, I think we do need to set defaults in TaxTypeManager. I was getting errors that "weight" could not be found without that default set.

We don't need $defaults, the warnings disappear when you clear cache.

Here's the final patch. Added a getter for the weight, and a test for it. We don't have plugins with different weights, so I can't expand OrderIntegration test to cover that right now.

Commited. Just in time for 2.16 :)