"Import add-on" button restyle

#### Purpose

Rename "Import add-on" button to "Add". Restyle it.

#### Modifications

Restyled button in XAML. Added context menu for button. New view:

##### A
![001](https://cloud.githubusercontent.com/assets/8158551/5316523/3a21fdea-7c95-11e4-9207-038905c9c85c.PNG)

##### B

![001](https://cloud.githubusercontent.com/assets/8158551/5338632/d75e9514-7ede-11e4-92e5-a64aa3c5479c.PNG)

#### Additional references

[MAGN-5599](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5599).

#### Reviewers

@ramramps, please, take a look.
@Benglin, FYI.
Just want to make sure we want ```e.Handled = true``` for all the mouse-down message.