ProjectExplorer: Prepare more flexibility to aspect layouting

This hides the explicit use of a QFormLayout from the aspect
interface in a new LayoutBuilder class. That currently works
only on a QFormLayout in the back, but opens the possibility
to use e.g. a QGridLayout as use on the Kits and some option
pages.

The aspects now only announce sub-widgets they like to add,
actuall positioning is does by a new LayoutBuilder class,
also cramming several widgets in an hbox in the right column
of the QFormLayout is done there.

Change-Id: I2b788192c465f2ab82261849d34e514697c5a491
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/android/adbcommandswidget.h

@@ -39,7 +39,7 @@ class AdbCommandsWidget : public QGroupBox
 {
     Q_OBJECT
 public:
-    explicit AdbCommandsWidget(QWidget *parent);
+    AdbCommandsWidget();
     ~AdbCommandsWidget() override;
 
     QStringList commandsList() const;