Extended Selection & Deletion of Path Points

[RobertBColton]

• Overload RowRemovalOperation::RemoveRows to accept a QModelIndexList of all selected rows.
• Set QAbstractItemView::ExtendedSelection on Path editor points list view so we can select to delete many points at once. This does not change the fact that there is a current index inside the selection which has the focus if an edit is requested.
• Set QAbstractItemView::SelectRows on Path editors points list view so we are actually selecting the entire point, which is similar to GM/LGM and feels less awkward. There is still a current index inside the selected row which has the focus if an edit is requested in a specific column item (aka the field).
• Use the new RowRemovalOperation overload to bulk delete all the selected points in the path editor. I used QItemSelectionModel::selectedRows so that we only delete points where every column (aka the entire row) is selected. This should always be the case since I've also changed the selection behavior to select rows instead of items.
• Remove selection after setting the current index, which already does a correct selection. As I've stated, the current index resides in the selection and belongs to the selection model.
• Add clarifying comments to the selection update slot about interaction with the above changes.
• The selected indexes passed to selection change slot may be empty and yet there is still a selection and so enabling of the delete button now needs to check the global selection model of the points table to determine if anything is selected. It should also check if there are selected rows and not just selected items.
• The selected point in the preview area is now the last, most recently selected point in the global, cumulative list selection.
• Deleting a point now keeps the same row selected, unless the last point is being deleted. This is how GM and the main Qt examples work with as well. Bounds checking is done against the row count so when the last point is deleted we do go to the previous row.

Understand that I am still not entirely sure the interaction of this with the current index. It seems that it is possible for the current index (which has the focus?) to reside outside of the selection if we deselect the current index. I am not sure how this interacts with QTreeView::allColumnsShowFocus but I suspect it's related. Regardless, other than being weird and not sure of the expected behavior, it doesn't seem to cause any issues.

In a view, there is always a current item and a selected item - two independent states. An item can be the current item and selected at the same time. The view is responsible for ensuring that there is always a current item as keyboard navigation, for example, requires a current item.
https://doc.qt.io/qt-5/model-view-programming.html#current-item-and-selected-items

[RobertBColton]

Ideally and eventually I want these previews to share a selection model with the list views and stuff. The selected points should be the same as those selected in the list so they can be moved together logically. We should also probably be painting the selection using the current style so it's consistent with platform conventions. The views should also maybe maintain a current index with focus indication like QGraphicsView/QGraphicsScene does.
https://doc.qt.io/qt-5/qtwidgets-itemviews-chart-example.html

[JoshDreamland]

Thought this was going to be a for-loop, now. Write it like this, please:

RepeatedMessageModel::RowRemovalOperation(_pointsModel)
    .RemoveRows(_ui->pointsTableView->selectionModel()->selectedRows());

[RobertBColton]

Aight 25f71db and it is a for loop, it's a for loop in the class so we don't duplicate it everywhere.
https://github.com/enigma-dev/RadialGM/pull/179/files#diff-9223a9db2dfd4809200a0a8574385ed1R147