Updates translations installation and search paths

liblxqt translations paths are relative to ${LXQT_TRANSLATIONS_DIR}.
liblxqt translations are installed to liblxqt/

Translations base search paths are:
	LXQT_SHARE_TRANSLATIONS_DIR
        XdgDirs::dataDirs(LXQT_RELATIVE_SHARE_TRANSLATIONS_DIR)
in that order.

For each directory in the base search path, the translation file, is first
searched inside a directory with the name of the app or library (without
version suffix) then in the base search path directory. Ex:
	/usr/share/lxqt-qt5/translations/liblxqt/
	/usr/share/lxqt-qt5/translations

LxQt::Translator class can be used to load the applications translations.
That would ditch the TRANSLATE_APP macro, that we use in each executable,
thus reducing resource consumption.
Adds Qt5 automatic load of self translations

loadSelfTranslation() will be called from the QCoreApplication constructor.
This way we can load the translation without requiring the application to
call it. It piles up, we can have all libraries translations self loaded
with Q_CORE_STARTUP_FUNCTION macro. It works for Qt 5.1 and later.

Sadly such a macro isn't available for Qt4.
Adds a translation loader for plugins

Plugins are different, they have a type or owner. The translations are
installed to a owner subfolder. The logic that applies to libs and apps
fails with plugins.
We search for the translations in /pluginnOwner/pluginName relative to the
base search path provided by getSearchPaths();
Ex: /usr/share/lxqt-qt5/translations/lxqt-panel/quicklaunch/
lxqt-panel is the type and quicklaunch is the plugin.