[Lazarus] Mouse multi click rewritten

[Ondrej Pokorny]

On 29.02.2016 9:27, Péter Gábor wrote:
> I'm on Linux / GTK2 and today I found that since r51723 the popup menu
> of Form designer does not appear on right click.

Thanks for reporting. I have overseen that Gtk2 indexes the mouse 
buttons in a different way ((l, m, r, x) instead  of (l, r, m, x)). So 
every right click became middle click and vice versa :)
Fixed in r51750, please test.

Ondrej