Allow code model hinting via pointing active target

In complex projects some file can be present in multiple targets and builds with different options. This options can affect file parsing by the code model.

Currently code model has ability to divide files by targets. But there is no way to point active target and some times incorrect target and build options selects.

There is two ways to provide hint to the code model:

1. By default, use current run target
2. Add selector like '#' - "Additional C++ Preprocessor Directives" but not editor-wide - project wide, for example - near Locator bar.

First item can be used by default when applicable (it is not applicable for libraries). Second item can contains special item "Default" that implies current default behavior, other items - targets taken from the build system, that forces build params for files that presents in multiple targets.

Forced active target can be ignored for files that presents only in one target and there is no conflicts.

Note, some IDE like Clion implements such features already.