#17 "Called functions tree" could flag external definitions

[Yann Dirson]

When a called function is not defined within the
project (eg. my project is a sub-directory of a larger
project, namely something below linux/drivers/), it
would be nice to show it differently than other called
funcs which are part of the project. Avoiding the
needless "+" unfolding icon would be a start (although
it wouldn't help to distinguish funcs defined in the
project, but not calling anything).