Change the logic of secondaryMembers list population

#### Purpose

implement more elegant solution of show/hide "More" button.

#### Modifications

`HiddenMembers` list is removed. Functions `OnMoreButtonClick` and `TruncateSecondaryMembers` are rewritten to more elegant view. Property `IsMoreButtonVisisble` is changed manually.

#### Additional references

It was decided not introduce `StandardPanelViewModel`. But other comments of https://github.com/Benglin/Dynamo/pull/51 is addressed.

#### Reviewers

@Benglin, it is hard to use `ShowExtendedMembers` in simple way as you proposed in https://github.com/Benglin/Dynamo/pull/51#discussion_r17583342. because we don't know for which collection the button should be add. It can be any of three collections. I am proposing similar solution but logic actually situated in two methods not lists` properties. Please, take a look.
These look great!

Another unrelated thing: please create a new YouTrack task to get rid of ```SearchElementGroupToHeaderConverter``` and ```SearchElementGroup```. Then turn ```PrimaryHeaderGroup``` into a ```string``` property, set its value directly as string (without using any converter). For example, set it to ```StandardPanel.ActionHeaderTag```.