Check if bridge name is registered before erasing from bridges map.

On some platforms, calling erase when the bridge map is empty causes odd
side effects, typically a large binary dump to the console.
Merge topic 'john/bridge-erase-fix'

16a70dac Check if bridge name is registered before erasing from bridges map.
Add SMTK_ENABLE_DOCUMENTATION option

Handy for superbuilds, if nothing else
Merge topic 'john/enable-documentation-option'

2f873b2a Add SMTK_ENABLE_DOCUMENTATION option
Add Superclass typedef macro and use it on Bridges.

This also cleans up the bridge classes that had manually
implemented a static create() method.
Operators now return 'entities', not 'bodies'.
Delete property data when last property disappears.
Add "any" keyword to possible associations.
Support universal operators; add one (set property).

This commit adds support for "universal" operators that can work on any bridge.
It is accomplished by allowing operators to be registered with the base Bridge
class and adding the inheritsOperators() method to bridges so that remote
bridges can avoid inheriting any operators from base classes.

This commit also adds a new operator that is (suprise!) universal:
the "set property" operator.
It sets property values such as the name and color on model entities, which
is typically information not stored in the foreign modeling kernel.
Merge topic 'universal-operators'

7b117dc8 Support universal operators; add one (set property).
6add2897 Add "any" keyword to possible associations.
b55330b4 Delete property data when last property disappears.
c5745a51 Operators now return 'entities', not 'bodies'.
d3633d4c Add Superclass typedef macro and use it on Bridges.
Fix MSVC's inability to handle temporary objects.
Merge topic 'universal-operators'

c968d570 Fix MSVC's inability to handle temporary objects.
More information in the bridge tutorial.
Amend for changes to bridge.
Fix the test; avoid duplicate variable name.
Add some meat to the bridge tutorial.

Also, put the "first steps" tutorial before the more advanced
bridge and operator tutorials.
Merge topic 'bridge-tut'

66b76c35 Add some meat to the bridge tutorial.
5cdaa9ed Fix the test; avoid duplicate variable name.
67379c72 Amend for changes to bridge.
47fe29f0 More information in the bridge tutorial.