Added a StubIndex for RobotKeyword type (keyword usages.)
Modified existing find usages code to simply use this index.

Improved parsing in a few situations (and added unit tests):
-Can have junk after a table heading
-Can start a Test Case on the same line as the test case header
-Can have Test Cases with names that happen to be the names of Settings,
e.g. a test case called "[tags]".