Maximise button refinement

[imranmunshi]

The theme is amazing, I finally switched from Zukitre to Plano. Great work.

It would be great if the maximize/unmaximize button could have different symbols/glyphs for maximize and unmaximize.

Right now, whether it is teh CSD (gtk3) or xfwm, the maximize button remains the same irrespective of whether the window is maximized or not. It should rather show a different glyph/symbol when a window is maximized and the normal glyph when the window is floating.

The 2 arrows pointing away could point towards each other when the window is maximized, for example.