Fixes Issue 2934 Fixes Issue 2935

Provides for storing Folder Settings in the central Preferences
Storage as a back-up to the settings stored on each folder.  In this
way, even if the LocalStore DB is recreated or otherwise lost, Folder
Settings can be recovered.

1) Does not change the methodology used to read settings while
running, nor the changes in r3107 & r3116 which tremendously improve
Accounts list loading time.

2) Loads Folder Settings from Preferences and stores on the folder
only when creating a new LocalFolder

3) Saves Folder Settings to Preferences and the DB row every time the
Folder Settings are changed.

4) When upgrading from DB version 41 to 42 or later, copies all
settings for existing folders from the DB storage to the Preferences
Storage.

5) Transactional bulk folder creation and single pass local folder
existence check during "Refresh folders" operation drastically reduces
time spent when refreshing folders from the remote store.

6) Uses prepared statement during Editor commit to reduce Preference
storing time.

Probably needs a reversion of r3239, but I'm unfamiliar with
translations, so am leaving that to others' discretion.