Fix some bugs in QQuickLinearLayout::insertLayoutItems()

* Fix alignment support for Row/ColumnLayout.
* Ignore "Repeater-like" items just like QQuickGridLayout

test_alignment() tested actually the gridlayout, therefore move it to
tst_gridlayout.qml.
Add a RowLayout-specific autotest.

Change-Id: I5e32775cf7f533f91ce68f8ec9fd95978cda7005
Reviewed-by: Caroline Chao <caroline.chao@digia.com>