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/ui/template/plugin.py

@@ -0,0 +1,40 @@
+# Python imports
+
+# Lib imports
+import gi
+gi.require_version('Gtk', '3.0')
+from gi.repository import Gtk
+
+# Application imports
+from libs.dto.base_event import BaseEvent
+
+from plugins.plugin_types import PluginUI
+
+
+
+class Plugin(PluginUI):
+    def __init__(self):
+        super(Plugin, self).__init__()
+
+
+    def _controller_message(self, event: BaseEvent):
+        ...
+
+    def load(self):
+        ui_element = self.requests_ui_element("header-container")
+        ui_element.add( self.generate_plugin_element() )
+
+    def run(self):
+        ...
+ 
+    def generate_plugin_element(self):
+        button = Gtk.Button(label = "Hello, World!")
+
+        button.connect("button-release-event", self.send_message)
+        button.show()
+
+        return button
+
+    def send_message(self, widget = None, eve = None):
+        logger.info("Hello, World!")
+