Provide option to set WindowBorderColor as non-gradient

[ThiconZ]

Right now the only option for setting a Window Border Color is through the Gradient fields. While the same color can be provided to both, it may be "simpler" to allow specifying a single color directly to a Theme.WindowBorderColor field and allowing it to be drawn using ImGui.GetWindowDrawList().AddRectFilled()

https://github.com/themeldingwars/ImTool/blob/master/ImTool/Window.cs#L222

[freakbyte]

I'm unsure about this as it would introduce extra complexity, just to save one line in a theme file.
What if both are specified in the themes file, do we use the solid color or gradient variable?