Added Visual markers to C and C++ snippets

This adds visual markers to C and C++ snippets where (I think) it makes sense. The way I've added them is a little weird. It looks like this for most

${VISUAL}${0:${VISUAL/(.*)/(?1::\/* code *\/)/}}

The reason for this is I wanted the $0 tabstop to have no default text when the VISUAL marker contains text and otherwise have the default text /* code */. This is the only way I could come up with to do this. If there is a better way, let me know.

also changed ife snippet to have tabs instead of spaces