Clean up deprecated code and fix multiple issues

- Remove deprecated markdown_preview plugin and re-wrote
- Renamed alt_provider.py under words completer
- Fix words completion provider logic and cache handling
- Fix container orientations (VERTICAL -> HORIZONTAL) and add Separators
- Remove unused Gtk imports from search_replace plugin
- Fix event creation parameter order in source_file.py
- Fix typo in source_view.py (_load_pretify_json -> _load_prettify_json)
- Refactor webkit_ui with new load methods and enhanced settings

plugins/code/markdown_preview/markdown/extensions/extra.py

@@ -0,0 +1,66 @@
+# Python-Markdown Extra Extension
+# ===============================
+
+# A compilation of various Python-Markdown extensions that imitates
+# [PHP Markdown Extra](http://michelf.com/projects/php-markdown/extra/).
+
+# See https://Python-Markdown.github.io/extensions/extra
+# for documentation.
+
+# Copyright The Python Markdown Project
+
+# License: [BSD](https://opensource.org/licenses/bsd-license.php)
+
+"""
+A compilation of various Python-Markdown extensions that imitates
+[PHP Markdown Extra](http://michelf.com/projects/php-markdown/extra/).
+
+Note that each of the individual extensions still need to be available
+on your `PYTHONPATH`. This extension simply wraps them all up as a
+convenience so that only one extension needs to be listed when
+initiating Markdown. See the documentation for each individual
+extension for specifics about that extension.
+
+There may be additional extensions that are distributed with
+Python-Markdown that are not included here in Extra. Those extensions
+are not part of PHP Markdown Extra, and therefore, not part of
+Python-Markdown Extra. If you really would like Extra to include
+additional extensions, we suggest creating your own clone of Extra
+under a different name. You could also edit the `extensions` global
+variable defined below, but be aware that such changes may be lost
+when you upgrade to any future version of Python-Markdown.
+
+See the [documentation](https://Python-Markdown.github.io/extensions/extra)
+for details.
+"""
+
+from __future__ import annotations
+
+from . import Extension
+
+extensions = [
+    'fenced_code',
+    'footnotes',
+    'attr_list',
+    'def_list',
+    'tables',
+    'abbr',
+    'md_in_html'
+]
+""" The list of included extensions. """
+
+
+class ExtraExtension(Extension):
+    """ Add various extensions to Markdown class."""
+
+    def __init__(self, **kwargs):
+        """ `config` is a dumb holder which gets passed to the actual extension later. """
+        self.config = kwargs
+
+    def extendMarkdown(self, md):
+        """ Register extension instances. """
+        md.registerExtensions(extensions, self.config)
+
+
+def makeExtension(**kwargs):  # pragma: no cover
+    return ExtraExtension(**kwargs)