Adapt to the translation infrastructure

Adapt to the changes in liblxqt. We now use the LxQt::Translator class to
load the translations. We save a few bytes ditching lxqttranslate.h
Load the plugins translations

This wasn't being done at all.
Just use the new LxQt::Translator::translatePlugin() to do the job.
Popup the configuration dialog when the user adds a panel

If the user added a new panel it probably wants to configure it. Just popup
the configuration dialog.
Panel background configuration

Options to set background color, image, or get from theme