Make UI more informative and save to opened file

[TsukiGva2]

Tabs are just placeholders atm, changing "Label" property should work

Also, "Save File" button should keep saving to the same opened file name

at notebook.lua

Change the "label" property to include the file name somehow
(suggestion: create a "file" property in the TextEditor instance)

local tab_label = Gtk.Label({ visible = true, label = "Tab"})
	local close_button = Gtk.Button({
		visible = true,
		relief = Gtk.ReliefStyle.NONE,
		Gtk.Image({ visible = true, icon_name = "window-close-symbolic"})
	})
})

at text_editor.lua

suggested changes:

+    function TextEditor.new(content, file_name)
	local self = {}

	self.content = content or ""
+       self.file_name = file_name
...