Bugfix: Fix config migrations from versions 2.8 to 2.9

Due to a value change of an internal Qt configuration variable, the configuration data could not be migrated on many
systems. We fixed this by implementing an additional migration path.

Furthermore, we removed the dependency on said value within the GUI, and use the values explicitly from the theme to
display the correct values on UI elements such as buttons.

https://github.com/owncloud/client/issues/8824
https://github.com/owncloud/client/pull/8860
