Fix and improve the color annotations widget

Fix: Global modifications in pqColorAnnotationsWidget (visibility and opacity values set in the header) now apply only on filtered items.

Changes :

• The filtering / sorting section of the the context menu (see pqTreeViewSelectionHelper) now appears only for filterable / sortable items.

• The opacity values of highlited items are set the same way the ckeck states are (via the context menu).

• As the opacity calues of highlited items are set via the context menu, the global opacity dialog of the pqColorAnnotationsWidget appearing when double-clicking on the table header doesn't fulfill this role anymore.