Replace Travis CI configuration with GitHub Actions workflows.
Add CI build workflow for Debian, Fedora, Ubuntu, and Archlinux.
Add release workflow for automated GitHub releases on tags.
Add dependabot configuration for GHA pin updates.
Remove obsolete .travis.yml.
remove .build.yml
fix invest applet makefile

* 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).

closes https://github.com/mate-desktop/mate-applets/issues/297

and require mate-panel >= 1.17

WARNING: use GTK+3 build of libmateweather for this build.
that lib is not migrated to GTK+3 only as we will possibly
use libgweather instead of it.