Change `<c-a>` to `<c-enter>` in Hints mode

Change `<c-a>` to `<c-enter>` in Hints mode

Using `<c-a>` for marking _all_ elements was nice, because `<c-a>` usually means
"select all". However, it conflicts with the "hold ctrl when typing the last
hint character to toggle if the link opens in a new tab or not" feature. If the
last character was an "a" that wouldn't happen; instead, the `<c-a>` command
would be invoked. This commit changes to `<c-enter>` which should be conflict
free.