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]".
Fixed parser so that characters before the first Robot Table are

treated as Comments, since that's how Robot works.
Minor fix to robot.flex. Just want a clean working tree before

starting on a larger task.
Fixed subtle issue with parsing argument list in the [arguments]

setting for a Robot Keyword Definition.

-Allow more characters for Robot Keywords, e.g. a Keyword can be a number (1234),
oddly enough.
-Add unit tests for these edge cases.
-Clean up parser's use of Robot variables as prep for doing variable autocomplete.
Autocomplete variables in scope-- from current test case / keyword definition +
and from all Variables Tables in current Robot file.

TODO: Would be best to also find variables from included resource files.