gtk-youtube-viewer: add option mainw_centered (default 0) to set the …

[tehlux]

…initial window position to 'center'

When starting gtk-youtube-viewer the window is created where I click the launcher (dock or panel) or run where the command (terminal). My personal preference is to have it start centered on the screen I usually just manually add a line to gtk-youtube-viewer after installation, where I set the initial position of the window to "center"

$mainw->set_position("center");

I don't know whether this is appreciated or considered useful or bloat but I thought I'd create a pull request which adds a new config option mainw_centered (0 or 1, defaults to 0) and have the centered position set after the window is resized to the size given in the config file (after which it is rerendered $mainw->reshow_with_initial_size;).

Feel free to reject.

[trizen]

I think it may be useful for more people, so I merged it. Thank you very much for your contribution.

[tehlux]

thank you!