Use newer "Segoe Fluent" icons instead of older MDL2

[bonzibudd]

Newer Windows builds have the simplified icon set. Fluent-Metro should be updated to match the new style.

[bonzibudd]

Here's a preview of a couple new icons. Also tweaked the Programs icon to have a back arrow after clicked and removed the blue highlight.

[bonzibudd]

I recently found an issue with the colorization of the icons, it appears that Windows uses a different line width depending on the color of the glyph, or uses a color somewhere in between white and black when drawing the icons. I am keeping the base icon completely black for now, but I might tweak them to be a dark gray instead.

[bonzibudd]

Addressed #57 in release 1.3.1. Still need to add some more small icons, but that won't be a noticeable change for the most part.