Added Insert Line Above/Below actions to BaseTextEditor

The actions perform exactly like their counterparts in Eclipse: a new,
indented line is inserted above or below the current line and the text
cursor is moved to the start of the new line. The line where the cursor
was before the action was triggered remains unchanged.

The shortcuts are also like in Eclipse: "Ctrl+Shift+Return" inserts a
line above the current, "Shift+Return" inserts a line below the current.

Merge-request: 140
Reviewed-by: Thorbjørn Lindeijer <thorbjorn.lindeijer@nokia.com>