ProjectExplorer: Dynamic padding for mini targetselector

ListWidget::padding() returned a contant 30, which can be too small if
the style has wider scrollbars than that.

This patch add pixel metrics from the style to roughly calculate the
needed padding, and adds 10 pixels to it.

Change-Id: Ia9b54174f9a91dee80ad99b4d9ad2489cde220c5
Reviewed-by: Daniel Teske <daniel.teske@digia.com>