* invest: Add chart with stock price history

Adds a chart window that shows stock price history over a configurable
time scale.

* invest: Use array index for x-axis

This gives all data points equal spacing and so gaps between market
closures are not shown.

* invest: Clean up chart pointers on applet removal

Running in-process mode, when the applet was removed from the panel, it
would try to call gtk_widget_get_visible on a destroyed widget, causing
a NULL pointer error. We fix it by properly cleaning up pointers on
applet destroy.

The original invest applet was dropped in
https://github.com/mate-desktop/mate-applets/commit/41f7b7a2a38ca9f3929fcbd88d228b3815034876
due to a broken API. This brings it back using the modern JSON version
of the Yahoo Finance API, a rewrite from Python to C, and a simplified
interface (i.e. instead of the full portfolio management app,
I simplified it into a stock ticker price tracker).

Don't use that value uninitialized

As Stickynotes doesn't seem to work with GtkSourceView anymore

*Otherwise we get segfaults