Because there are two dialogs for editing optional downloads, refactor
the separate implementations to select them into a common reusable widget
and use it in SelectiveDialog and InstallDialog.
Currently this is achieved by toggling the checked state twice
in a programmatic way. This ensures correct animations until
a better solution is found.
In addition, now both CollapsibleFrame and CollapsibleGroupBox
inherit their common methods from the abstract class CollapsibleBase.
This change makes it more inline with how QScrollArea operates on a central widget.
Other changes include using a QFrame instead of a QWidget as a base and
adding a QLabel for the title instead of the horizontal line.
The advanced options were split into their own separate widget. Right now
their class operates only as a container with the logic remaining in the
InstallDialog.
