Fix for Github issue 741: Add context menu for CMakeLists

[SchweizS]

Changes

• Added context menu entries for CMakeLists.txt in Explorer and CMake project outline.
• Added context menu entries for source files in CMake project outline.
• Removed context menu for all other file types (they all had "compile file", which resulted usually in compiler errors)
• Compile file only works if filename contains .c (e.g. .c, .cpp, .cxx)
• Added configure all projects to CMakeLists.txt files as icon in CMake project outline.

This should resolve #741

[bobbrow]

Thank you for another great PR! If you can address my one comment in tree.ts I'll merge this in.