Default icon for cusom node is different

#### Purpose

For custom nodes show different default icons.

#### Modifications

`loadDefaultIcon` is made as `protected virtual` - it redefined in `CustomNodeSearchElement`. Only for them we are showing different default icons.

Result is presented on screenshot:
![005](https://cloud.githubusercontent.com/assets/8158551/5224804/35153cfc-76e3-11e4-8143-72a22e8769e8.png)

#### Additional references

[MAGN-5548](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5548).

#### Reviewers

@ramramps, please, take a look.

@Benglin, FYI.