Hidden advanced preferences

This is a suggestion.

There are many requests to make Owncloud for Android more configurable. However, preferences menu should be as lean as possible, if I understand right.

How about a hidden preference view for advanced users? And if something goes wrong, these advanced prefs can be reset easily.

As an example I added a configurable upload path as suggested/requested by https://github.com/owncloud/android/pull/636, https://github.com/owncloud/android/issues/457, https://github.com/owncloud/android/issues/332, https://github.com/owncloud/android/issues/201, ...