Improve roundtrip of tex2lyx test documents:

Improve roundtrip of tex2lyx test documents:
- Make test-insets.tex and test-structure.tex compilable
- Avoid duplicate definition of \lyxarrow in test-insets.lyx
- Prevent subscript package from being ignored in test-insets.lyx
- Prevent commands listed with optional arg in syntax.default from being
  concatenated with the next word if no optional arg is given
- Handle spaces and comments inbetween a command an "{}" consistently

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@36943 a592a061-630c-0410-9148-cb99ea01b6c8