Change the way of switching between LibraryView and LibrarySearchView

#### Purpose

Modify switching between views in `LibraryContainerView` to the way which is proposed in https://github.com/Benglin/Dynamo/pull/38#discussion_r17294613.

#### Modifications

Added `CurrentMode` to `SearchViewModel`. Created converter `ViewModeToVisibilityConverter` which do all work.

#### Additional references

[MAGN-4530](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4530).

#### Reviewers

@Benglin, please take a look.