[mika76]

Adds three line commands to AvaloniaEditCommands, next to DeleteLine. Closes #317.

Command	Default shortcut	What it does
MoveLinesUp	Alt+Up	Moves the caret line, or the lines touched by the selection, up by one line
MoveLinesDown	Alt+Down	The same, down by one line
DuplicateLines	Ctrl+Shift+D	Inserts a copy of those lines below them

• The caret and selection stay on the moved or copied text, so the commands can be repeated.
• A last line with nothing selected on it isn't included, as with the other line commands.
• Each command is one undo step.
• Newlines are kept, and a copy of the document's last line uses the document's newline.
• Read-only sections are respected: nothing changes if the text isn't writable.
• Alt+Up and Alt+Down are free (box selection uses Alt+Shift on Windows and Ctrl+Shift on macOS). Ctrl+D is already DeleteLine, so duplicate is Ctrl+Shift+D.

About the default shortcuts: like DeleteLine and IndentSelection, the gestures are set on the commands, so every editor gets them. An application that already uses these keys keeps working if it handles them first, or it can remove the command bindings. If you'd rather these commands had no default gesture and left the keys to each application, that's an easy change.

Tests in LineCommandsTests cover moving and duplicating at the first and last line, with and without a final newline, with CRLF, with selections (including one that ends at the start of a line), empty lines, undo, read-only documents, and the default shortcuts.