Modify colors managment

Issue : #17 Default colour is white

Default color :
- 1-White, 2-Yellow, 3-Red, 4-Magenta, 5-Cyan, 6-Green
- These colors can be modify by right clicking -> "Change color..."
- The color number 7 is changed when the current line color is not in the
palette.


Signed-off-by: juiice <decaen.julien@gmail.com>