GUI: Make GridWidget accept focus

This avoids the filter text to take the focus.
In Android this prevents the launcher to start with the virtual keyboard
displayed.
In the future, the widget could also take keyboards event into account
like the ListWidget one.

gui/widgets/grid.h

@@ -206,6 +206,8 @@ public:
 
 	void reflowLayout() override;
 
+	bool wantsFocus() override { return true; }
+
 	void openTray(int x, int y, int entryID);
 	void openTrayAtSelected();
 	void scrollBarRecalc();