texteditor: allow quickfixes also in overwrite mode

We now temporily switch to insert mode instead. Completely blocking
would break quickfixes in fakevim, as it keeps the texteditor in
overwrite mode most of the time.

Change-Id: Ibc492c80bf02fd2a203a607edc6966c338854a9f
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>