Fix bug where post-execute code was being called twice.

Renamed 'payload-svg' matplotlib backend to 'inline'.

* Added Cut support to ConsoleWidget. * Added Raw Copy to FrontendWidget. * Minor cleanup

Note Consolas on linux will be used only until we have font configuration.

Add paste to plt, fix spurious newlines in cell mode.

Add support for simultaneous interactive and inline matplotlib plots. In purely svg mode, there is now no lag with draw() operations, they just produce the figure immediately. And in gui mode, a new function, 'paste', is added to plt and to the user's namespace, that pastes open figures from GUI windows into the editing widget as inline SVGs.

Centralize Qt font selection into a generic utility. This makes a more flexible choice between main requested font and fallbacks, using a QFontInfo object that knows about the local platform. Use 'Consolas' on Win32 and Linux if available, fall back to respective native fonts otherwise.

Added support for Ctrl-Enter (unconditionally enter a newline) to ConsoleWidget. This works at the ConsoleWidget level, but it can confuse InputSplitter to create some unintuitive auto-execution results. InputSplitter will probably need some fixes.

First cut at support readline-esque history filtering in ConsoleWidget.

Moved HistoryConsoleWidget to its own file to make console_widget.py a little smaller.