#889 Add a terminal tab in lower sidebar

[J B]

Many time one needs to run a quick command on the terminal with TeXing. E.g. delete something, move a file or something else.

Thus, a terminal window in the lower sidebar next to the "Messages" "Log" "Preview" tabs would be nice. Eg. if there is an error message about something external, one could fix it from the terminal right away.

[Tim Hoffmann]

duplicate [feature-requests:#721]