Added API to get all scene plugins

This complements the API to get the active scene plugins.

Change-Id: I652d0da72058d7ca95360fa443c2a21fefce933e
Added new signals to the extension plugin class

New signals enable the extension to request the active tool, and the
active display type(s). These can then be used by the application to
change tools and display properties using the plugin names.

Change-Id: I290e297a8ee1b22014e5de9a74a97fb063355ab5
Merge topic 'more-extension-signals' into master

1eac9f4c Added new signals to the extension plugin class
02eff5d3 Added API to get all scene plugins