Improve backup error handling and frequency

This patch improves our backup functionality in a number of ways:
- Only backup the vault when important changes are made, not when the order of
  entries is changed, for instance.
- Don't bubble up backup errors when saving the vault.
- Instead, show an error bar in the main view if the most recent backup attempt
  failed.

<img src="https://alexbakker.me/u/kbhhj2hcgx.png" width="300" />

Clicking on the error bar will take the user to the backup settings.

app/src/main/java/com/beemdevelopment/aegis/ui/AegisActivity.java

@@ -122,9 +122,9 @@ public abstract class AegisActivity extends AppCompatActivity implements AegisAp
         this.getResources().updateConfiguration(config, this.getResources().getDisplayMetrics());
     }
 
-    protected boolean saveVault() {
+    protected boolean saveVault(boolean backup) {
         try {
-            getApp().getVaultManager().save();
+            getApp().getVaultManager().save(backup);
             return true;
         } catch (VaultManagerException e) {
             Toast.makeText(this, getString(R.string.saving_error), Toast.LENGTH_LONG).show();