Draft: Preference: Zoom the font only
This MR meets the requirement of issue #321
- Added a preference entry: when zooming in or out, don't change window's size.
- Fixed a typo in terminal-preferences.c
- Added TODO comments. I tried to make it performs like Konsole (after zooming, the screen will display new terminal size like 120x29) but I don't know how to implement this feature.