Add sort oder indicators in main table

[tobiasdiez]

In the header of the main table, there should be little arrows indicating the active sort order of the column.

[matthiasgeiger]

See #609 - was present in earlier version, and should be generally possible to add it again - but the way it should work isn't working somehow...

Unfortunately I had no time to dig deeper into this lateley... 😞

[tobiasdiez]

This should be implemented in the latest development version. Could you please check the build from https://builds.jabref.org/maintable-beta/. Thanks! Note this version is in a very early beta stage and might not work as expected. See #3621 for further details.