DUI: tooltip for addons. Part 1

# Preface
Here is new tooltip, that should be shown for addons.
![image](https://cloud.githubusercontent.com/assets/8158404/4951728/f65c1d42-666a-11e4-908d-65d4f2c3fe99.png)

This PR is just the first part of tooltip implementation.

# What is done:
* I couldn't suggest, that we will have set of outputs... That's why instead of `string OutputParameters` we have to use `List<string> OutputParameters`.
* If element is custom node, output should have light grey color. For this purpose `CustomNodeToBoolConverter` was added.
* When these is no type of value (i.e. input parameters in custom nodes), we shouldn't add `none`. For this purpose `InOutParamTypeConverter` was modified.
* If there is no desciption, we have to write “No description available”.
* Addons text label color was changed to #666666.
* And tagetLocation.x (of custom node) can be more that 0. I mean next:
![image](https://cloud.githubusercontent.com/assets/8158404/4951879/c716bcb6-666c-11e4-876f-399d6e1da89d.png)
That's why we need to take in consider this blue part.

# Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5339](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5339) DUI: Show "No description available" for empty description
[MAGN-5338](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5338) DUI: Show new design tooltip for addons member

I may have asked once before, but just in case I have not, can we have ```OutputParameters``` property and ```GenerateOutputParameters()``` to return an ```IEnumerable<string>``` instead of ```List<string>```? Same question for inputs.