GUI: fix API executor look

[narnaud]

The API executor is a small widget that appears under the document (like the find widget), with Ctrl+Shift+R.
You can see some data if you open it with a C++ file.

Two main issues:

1. the menu is not there, it should be somewhere (maybe under File->Show palette...)
2. if you open it on a txt file (or any other file than a C++ file), then on a C++ file, the combobox with the function name is too small. It should adapt to the size of the items in it.