Viewrendered3 Gets A Dark Theme

[tbp1...@gmail.com]

I have created a pull request for the Viewrendered3 plugin.  It adds a new stylesheet for Restructured Text that has a solarized dark theme.  By default VR3 attempts to figure out if the Leo theme is a dark one, and will use the new stylesheet if the theme seems to be one (this is done in a simple-minded way).  This behavior can be overridden with a new setting.  The ability to specify a custom RsT stylesheet has been generalized so that both relative and absolute paths will be understood.

The new stylesheet is based on my own tweaking of Leo's current default theme.  The background is a little grayer than black, and the default font color is a little grayer than white.  This makes it look something like VSCode's default appearance.  It will also use the Cousine monospaced font that I recently discovered, if that's been installed.

There are more tweaks that could be made, so feel free to experiment.  The new capabilities are described in the docstring, which can be viewed using the menu item Plugins/viewrendered3/about.

This version will be available once it is merged into devel, or you can get it from my repo's tbp-freefloat-editor branch at tbp1's repo.  There is one new stylesheet in the plugins/viewrendered3 directory in addition to the new viewrendered3.py module itself.