Fix plugins so that they can be deleted without being initialized

Previously, several plugins would segfault if their destructors ran
without their onInitialize() method being called.  To fix this issue, add
a new method Display::initialized() which returns true if onDisplay()
has finished.  Subclasses can use this method to see if they need to
cleanup resources from their onInitialize() method.
Free dynamically allocated displays in AddDisplayDialog
Use DisplayFactory to create plugins in AddDisplayDialog
Speedup construction of image plugins

Refactor ImageDisplayBase to scan for transport plugins and create its
ImageTransport instance in onInitialize() instead of in the constructor.
This lets CameraDisplay and ImageDisplay be constructed much faster.
Speedup construction of depth cloud plugin

Move creation of the image transports and pointcloud_common from the
constructor to onInitialize().
Speedup construction of point cloud plugins + laser plugin

Move creation and enumeration of plugins from PointCloudCommon's
constructor to its onInitialize() method.