kdevelop: Update to 4.6.

This updates KDevelop and KDevPlatform to the latest versions.

To get it to run, the following is (and has been) required:

- Either install this and all KDE dependencies to user environment, or add this to configuration:
```
environment.pathsToLink = ["/share/apps" "/share/kde4"];
```
- Make sure `pkgs.kde4.kdelibs` is installed either in user or system environment (e.g. add it to `environment.systemPackages`). This is needed to get `kbuildsycoca4` in PATH.
- Run `kbuildsycoca4` once in user session (or re-login).