Add right click menu option for opening URLs using xdg-open.

[multiplexd]

Other terminal emulators such as xfce4-terminal have a right-click menu option which opens the URL under the pointer. This commit adds this functionality to lxterminal using a double-fork() and exec() of xdg-open.

Note: I am not familiar with the internals of lxterminal, my implementation is based on the corresponding code for copying a URL under the pointer.

[multiplexd]

When I opened this pull request I did not realise that lxterminal already implements the desired functionality except via control-click instead of a right-click menu option. I have rewritten the menu option using the control-click handler and replaced the previous hand-rolled fork+exec code. This means that a URL under the pointer may now be opened via two methods.

[medicalwei]

Thank you! Ctrl-click on the link is a "hidden" feature that we seldom advertise.