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/manifest.json

@@ -0,0 +1,7 @@
+{
+    "name": "Markdown Preview",
+    "author": "ITDominator",
+    "version": "0.0.1",
+    "support": "",
+    "requests": {}
+}