Autoresize table subview columns (bug #6939)

Should fix #6939 (closed) for standard record tables. Nested tables are already autoresized while other types of tables should look okay without autoresizing.
I decided to use Qt's content-based autoresize, but prevent it from making the column too short or too long by clamping the size hint - and taking way too much time by taking the contents of the first 500 records in the table (the number is arbitrary). The autoresizing happens when the table is generated. Unfortunately 500 records isn't nearly sufficient when there are so many different types of columns, so please check if the general feel of Objects table is okay-ish.