Various structure editing UI features

here is a grab bag of small UI issues related to the structure view:
- [ ] the delete button is very far away from the column name. Thus marking the right column for deletion can be hit and miss if the windows is big and there a a lot of columns. The button could be relocated or the column (or just the button) could be highlight while the mouse cursor is hovering over the column
- [ ] multi column primary keys should be represented in the constraints section
- [ ] indices could also have a section in the columns list; they could also have a colour coding so that multi column indices could be easily identified and the columns and index definition could easily be related
- [x] the column/content toggle button should not be reset if the table is changed. If I'm editing a table structure I am also likely to check/edit the table structure of another table. The current design makes this process quite tedious
- [ ] editing the type should bring up a small popup-window (like when editing foreign keys) with a dedicated `USING` field. While it's easy to use once you know that the `USING` clause can be written there, it is not very discoverable

Start by reproducing the unchecked items in the structure view, especially deletion targeting, composite primary keys and indices. Read the existing foreign-key editing flow and inspect how the column/content toggle behaves when switching tables. Done means the remaining checklist items are implemented and each interaction is verified in the structure editor.

Written by the indexing model from the issue text.