Improved Syntax Highlighting and added Symbol List

Hi Markus,

although, it is still a work in progress, I propose these changes for CMake language.  
It has many improvements,
- support for nested variables
- hightlighted options
- variables in strings
- escaped chars in strings
- started with contextual keywords (for if-family commands and file command)
- symbol list (macros, functions, project, add_executable and add_library)

best regards, Kiwi