Window size as command-line argument

[dbaelz]

Currently the initial window size for the floating window is hardcoded and therefore can't change after the app is build/distributed.

Enhancement: It would be great to have two command-line arguments to set width and height.
Example: demoapplication 1024 768 for a window with width 1024 and 78 height. Maybe another style (like an option --width=1024) is better. Feel free to discuss in here or implement it as you like.