Make loading of settings more robust

In general, this MR makes loading of settings more robust. This effort of @c-p and mine was originated by sardana-org/sardana#1716 (comment 742615517). But, we believe it may avoid other problems related with loading of settings. From now on, different configurable items e.g. widgets announcing supported config versions will be protected against loading incompatible settings.

Added:

• validation of config version
• make save settings on GUI closure optional
• TaurusMainWindow.SAVE_SETTINGS_ON_CLOSE which is None by default (to interactively ask user)

Changed:

• show TaurusMessageBox instead of QMessageBox for failed loading of settings to offer an easy issue reporting

Deprecated:

• __UNVERSIONED__ ConfigVersion - will be auto-fixed on next save of settings

Fixed:

• restoring config of (at least) TaurusForm by fixing order of storing model config properties in TaurusGui.createCustomPanel()
• show ConfigVersion in taurus config browser