Set default-location for projects to documents-dir.

All export files that are derived from the project (BOM,
nomenclature, etc.) are saved in the same directory by default.
In this context, the standard directories have been grouped
together in qetapp.cpp / qetapp.h so that only one place needs
to be searched for in case of any adjustments.

sources/qetdiagrameditor.cpp

@@ -65,7 +65,7 @@ QETDiagramEditor::QETDiagramEditor(const QStringList &files, QWidget *parent) :
 	m_zoom_actions_group       (this),
 	m_select_actions_group     (this),
 	m_file_actions_group       (this),
-	open_dialog_dir            (QStandardPaths::writableLocation(QStandardPaths::DesktopLocation))
+	open_dialog_dir            (QETApp::documentDir())
 {
 		//Trivial property use to set the graphics handler size
 	setProperty("graphics_handler_size", 10);