Do not resize new screen lines

Summary:
The lines are resized when something is put in them. Resizing them
here filled them with spaces (default value), later those trailing
spaces were put into history, and eventually into saved output or
clipboard (because it is assumed lines in history do not contain
trailing spaces).

BUG: 392419

Test Plan:
* Enable "Trim trailing spaces" in Profile Settings → Mouse
* Set window size to about 100x100px
* Clear terminal (Ctrl-Shift-K by default)
* Run:
```
seq $(( $(tput lines) * 2 )); \
printf '*** resize ***'; read -s; printf '\n'; \
seq $(( $(tput lines) * 2 ))
```
* When "resize" show up, resize the window to about 500x500px
* Press enter
* Select and copy lines below `*** resize ***`
* Paste it in a text editor which allows to show trailing spaces

Expected result: lines should not contain trailing spaces
Actual result: lines added when resizing the window have trailing spaces

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D11893

(cherry picked from commit 61069543)