Add infrastructure for displaying graphics in terminal

This allows for showing graphics in the terminal.
konsole keeps two lists of graphics to display (called placements).
Each placement includes:
- A pixmap to display.
- Column and row where the image is displayed.
- A z index. The text has z=0, so placements with z<0 are displayed below the text.
- An opacity value with which the pixmap is drawn.
- Whether the graphics scrolls with the text

Graphics placements with pid >= 0 are managed by the application, which can remove them. They need to have a unique <id,pid> pair.
Placements with pid<0 are not managed by the application. They are deleted automatically when they are completely obscured by others.