Create an about dialog for all LXQt apps

[miway88]

I see wich any application of LXQt have they own about dialog, i think wich with a general about dialog is more attractive and easy for the developer.

[tsujan]

Not all LXQt apps depend on liblxqt (pcmanfm-qt and QTerminal don't) and those apps that depend on it (like lxqt-config) may not need an about dialog.

[miway88]

Yes, but is good idea have one for an application wich need one, I can do it!