Window controls need updating to the latest style

The window control buttons need updating again, now that the rest of Salience has changed around them.

Planned changes:
* Remove the orange glow border when hovered with the mouse
* Bring the buttons closer together
* Edit the trough to maybe find a better gradient and have it blend into the rest of the titlebar better
* Update the buttons' symbols to match how text/icons on buttons appear elsewhere
* (Maybe?) change the background color of all buttons other than the close button to the same dark gray used by buttons within the window