Add multiline editing feature.

• Multiline Editing
• Keyboard interactions are inspired from QtCreator.
• Alt+Shift+[up or down] to edit upper or lower lines.
• Move editing cursors through: Ctrl+(up, down, left, right, end, home)
• Feature and unit test.

There are probably some unmanaged cases about selection, tabulation… but it is a good base. It may not be a good idea to merge it in master directly.