[resources/maildir] Reload configuraton on configuration change

Summary:
When user adds new maildir, a new resource gets created, with the default
directory ~/.local/share/local-mail. Since it's a new resource, with no
proper configuration, an attemptConfigRestoring() is called, which changes
it to ~/.local/share/akonadi_maildir_resource#. It's stored inside
mSettings->path().

Then a dialog appears, where user can choose prefered directory. It gets
written to the config file; then configurationChanged gets called.
We call mSettings->save(), which overwrites the path provided by user with
the default one (~/.local/share/akonadi_maildir_resource#), making it
impossible to create a new maildir anywhere else.

Just use load() instead, it makes more sense when configuration was changed.

BUG: 416287
CCBUG: 415245
CCBUG: 415922

Test Plan:

1. Create a new maildir resource pointing to /tmp/dummy.
2. The resource gets created, with the name dummy. /tmp/dummy gets created.
3. Drop some mails into it via KMail. Mails appear inside /tmp/dummy

Reviewers: dvratil, mlaurent

Reviewed By: dvratil

Subscribers: wbauer, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D27905