Add function fplWindowDialog() for all platforms

[f1nalspace]

Its nice for simple feedback to show messageboxes, so we need to add support for it.
For win32 its damn easy: MessageBox()

For X11 its a pain in the ..., because there is no such function. You have to create a new X11 window, create a label and button on it and display it. I am not sure if X11 has raw support for buttons and labels... so some sort of library are required (Gnome, KDE, etc).

References:

https://github.com/Eleobert/MessageBox-X11/blob/master/messagebox.c
https://rosettacode.org/wiki/Window_creation/X11#C