Remove tabs UI from code editor and move to plugin. Enhance plugin system.

- Remove tabs controller, tab widget, and tabs widget files and move to plugin
- Delete plugins/README.txt
- Add register_controller method to controller system for plugin use
- Add error handling for plugin crashes via futures callback

plugins/code/tabs_bar/tab_widget.py

@@ -0,0 +1,79 @@
+# Python imports
+
+# Lib imports
+import gi
+gi.require_version('Gtk', '3.0')
+from gi.repository import Gtk
+
+# Application imports
+
+
+
+class TabWidget(Gtk.Box):
+    """docstring for TabWidget"""
+
+    def __init__(self):
+        super(TabWidget, self).__init__()
+
+        self.file            = None
+
+        self._handler_id     = None
+        self._eve_handler_id = None
+
+        self._setup_styling()
+        self._setup_signals()
+        self._load_widgets()
+
+
+    def _setup_styling(self):
+        ctx = self.get_style_context()
+        ctx.add_class("tab-widget")
+
+        self.set_orientation(0)
+        self.set_hexpand(False)
+        self.set_vexpand(False)
+        self.set_size_request(-1, 12)
+
+    def _setup_signals(self):
+        ...
+
+    def _load_widgets(self):
+        self.event_box  = Gtk.EventBox()
+        self.label      = Gtk.Label()
+        self.close_bttn = Gtk.Button()
+        icon            = Gtk.Image(stock = Gtk.STOCK_CLOSE)
+
+        self.event_box.set_above_child(True)
+        ctx = self.label.get_style_context()
+        ctx.add_class("tab-label")
+        ctx = self.close_bttn.get_style_context()
+        ctx.add_class("tab-close-bttn")
+
+        self.label.set_xalign(0.0)
+        self.label.set_margin_left(25)
+        self.label.set_margin_right(25)
+        self.label.set_hexpand(True)
+
+        self.close_bttn.add(icon)
+        self.event_box.add(self.label)
+        self.add(self.event_box)
+        self.add(self.close_bttn)
+
+        self.show_all()
+
+    def clear_signals_and_data(self):
+        self.close_bttn.disconnect(self._handler_id)
+        self.event_box.disconnect(self._eve_handler_id)
+        self._handler_id = None
+
+        for child in self.get_children():
+            child.unparent()
+            child.run_dispose()
+            child.destroy()
+
+    def set_close_signal(self, callback):
+        self._handler_id = self.close_bttn.connect(
+            'clicked',
+            callback,
+            self.file
+        )