Improve table navigation with keyboard

• Adding a table at the very top of the document, I'm no longer able to put something on top if it afterwards. Would be nice if I could put the cursor above the table somehow
• Navigating with the arrow up (or left) key from below the table, the cursor ends up on the right side of the table. I'm able to add content there. That's unexpected and I'd consider it unwanted ;)
• Further navigating up with the arrow up keys, the lower right field first is reached first and then it goes up the very right column until the last body field. I'm not able to jump into the header field by navigating with the cursors.
• If the table is the first node in my document and I try to navigate further up from a header field or the last body field, the cursor jumps below the table.

Originally posted by @mejo- in https://github.com/nextcloud/text/issues/2235#issuecomment-1076578223

Also there is no way to access the add-a-row-buttons etc with keyboard navigation.

No files or tests are named. Start by reproducing the reported keyboard navigation with a table at the top of a document, then trace the table editor's cursor and control-navigation behavior. Done means users can move before and after the table, reach header and body cells predictably, and access add-row controls with the keyboard.