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/Preferences.java

@@ -160,6 +160,14 @@ public class Preferences {
         _prefs.edit().putInt("pref_backups_versions", versions).apply();
     }
 
+    public void setBackupsError(Exception e) {
+        _prefs.edit().putString("pref_backups_error", e == null ? null : e.toString()).apply();
+    }
+
+    public String getBackupsError() {
+        return _prefs.getString("pref_backups_error", null);
+    }
+
     public boolean isTimeSyncWarningEnabled() {
         return _prefs.getBoolean("pref_warn_time_sync", true);
     }