Display character size on dragging window edges

[Roshan-R]

Should resolve #443

things to address upon :

• At present, it connects the size-allocate signal of the grid to call the function to show size, this could probably be improved by connecting to some other signal
• It shows the character size upon first launch
• Improve Design

[jeremypw]

@Roshan-R : This now conflicts with master so needs attention. Also, the character size of the window will change when the font size is changed - this does not seem to handle that? @danirabbit Is this something worth pursuing? I am not sure whether a Gtk4 version of Terminal is on the roadmap but this solution probably will not work as is.