Reduce metrics updates from 4 to 1 when loading file

Reduce metrics updates from 4 to 1 when loading file

The number of metrics updates when loading file and showing it in a
new work area is unreasonable.

The first avoided call to updateMetrics() was an explicit resize in
BufferView::init(). Instead, an assertion is suppressed by exiting
early BufferView::processUpdateFlags() when BufferView::ready()
returns false. This is a new method introduced to factor in some
existing tests.

Two other metrics computations are avoided by setting the enclosing
View object busy() while creating the new tab. To make this work
properly, GuiWorkArea::scheduleRedraw has to return early in this
case.

When saving an unnamed document or invoking "Save as...", call
setBusy(false) earlier so that repainting occurs correctly.

Fixes bug #12976.