Moved plugins and refactor command system

- Moved plugins to apropriate sub folders
- Refactor command system with new add_command method and rename GetCommandSystemEvent to GetNewCommandSystemEvent
- Add RegisterCommandEvent for dynamic command registration
- Change footer container orientation to VERTICAL
- Add search-highlight tag to source buffer
- Add file change detection (deleted, externally modified) in source_file
- Add JSON prettify option to source view popup menu
- Enable hexpand on VTE widget
- Update plugins_controller_mixin to use widget_registry

plugins/code/completers/example_completer/provider.py

@@ -0,0 +1,76 @@
+# Python imports
+
+# Lib imports
+import gi
+gi.require_version('GtkSource', '4')
+
+from gi.repository import GObject
+from gi.repository import GtkSource
+
+# Application imports
+from .provider_response_cache import ProviderResponseCache
+
+
+
+class Provider(GObject.GObject, GtkSource.CompletionProvider):
+    """
+        This is a custom Completion Example Provider.
+        # NOTE: used information from here --> https://warroom.rsmus.com/do-that-auto-complete/
+    """
+    __gtype_name__ = 'ExampleCompletionProvider'
+
+    def __init__(self):
+        super(Provider, self).__init__()
+
+        self.response_cache: ProviderResponseCache = ProviderResponseCache()
+
+
+    def do_get_name(self):
+        """ Returns: a new string containing the name of the provider. """
+        return 'Example Code Completion'
+
+    def do_match(self, context):
+        # Note: If provider is in interactive activation then need to check
+        # view focus as otherwise non focus views start trying to grab it.
+        completion = context.get_property("completion")
+        if not completion.get_view().has_focus(): return
+
+        word = self.response_cache.get_word(context)
+        if not word or len(word) < 2: return False
+        return True
+
+    def do_get_priority(self):
+        """ Determin position in result list along other providor results. """
+        return 5
+
+    def do_activate_proposal(self, proposal, iter_):
+        """ Manually handle actual completion insert or set flags and handle normally. """
+
+        buffer = iter_.get_buffer()
+        # Note: Flag mostly intended for SourceViewsMultiInsertState
+        #       to insure marker processes inserted text correctly.
+        buffer.is_processing_completion = True
+        return False
+
+    def do_get_activation(self):
+        """ The context for when a provider will show results """
+    #     return GtkSource.CompletionActivation.NONE
+    #     return GtkSource.CompletionActivation.USER_REQUESTED
+    #     return GtkSource.CompletionActivation.USER_REQUESTED | GtkSource.CompletionActivation.INTERACTIVE
+        return GtkSource.CompletionActivation.INTERACTIVE
+
+    def do_populate(self, context):
+        results   = self.response_cache.filter_with_context(context)
+        proposals = []
+
+        for entry in results:
+            proposals.append(
+                self.response_cache.create_completion_item(
+                    entry["label"],
+                    entry["text"],
+                    entry["info"]
+                )
+            )
+
+        context.add_proposals(self, proposals, True)
+