Provide Config option to have Window start in Center Screen

[ThiconZ]

Currently the Window start location is defined in the first 2 fields of the Configuration.NormalWindowbounds Rect (50, 50) which is then used for the Window creation in Window.Backend at https://github.com/themeldingwars/ImTool/blob/master/ImTool/Window.Backend.cs#L81

It would be useful to add a new setting in Configuration to enable always starting in Center Screen. Sdl2Native.SDL_WINDOWPOS_CENTERED can then be passed into the new Sdl2Window call in place of config.NormalWindowBounds. X/Y. This would avoid the user having to always calculate the center point and allow SDL to handle the work for them.

[freakbyte]

Default behavior is to center the window on first launch now: e1e9675