ViewNavigator tree entries become bold when opened

Summary:
Rename plugin activator files

For a cleaner and simpler naming the files
were renamed.

Clean view navigator view files

In order to stick to the common naming convention,
the class "ViewNavigatorView" was renamed.
Additionally the "VIEW_ID" was changed according to the
common naming convention.

Correct view navigator item files

In order to stick to the common naming convention,
the classes "mitkQtViewItem" and "mitkQtPerspectiveItem" were
renamed to "QmitkViewItem" and "QmitkPerspectiveItem".
The class-names were changed in "QmitkViewNavigatorWidget" accordingly.

Clean view navigator widget files

Functions of the "ClassFilterProxyModel" class have directly
been defined inside the nested class to make the nested classes
more consistent.
Member variable and function names were changed according to the
common naming convention.
Functions have been reordered according to the access specifiers.
Unnecesary comments have been removed to clean the code.

Make class more readable

Use part listener to update tree on views status

If views are opened / closed, the tree list is updated.
The update function checks each tree item, compares them with
opened views and sets the font to bold if necessary.

Clean view navigator widget files

Loops have been modernized.

Test Plan:

1. open MITK workbench, open the ViewNavigator
2. double click on a tree item to open another view
3. see how the tree item is written with bold font
4. open a plugin view from the plugin menu bar
5. see how the corresponding tree item is written with bold font
6. close and re-open the view navigator
7. see how the previous state is restored
8. close a plugin view
9. see how the corresponding tree item is written with normal font

Reviewers: kislinsk, O1 MITK Reviewer Group I, O3 MITK Reviewer Group II

Reviewed By: kislinsk, O1 MITK Reviewer Group I, O3 MITK Reviewer Group II

Subscribers: kislinsk, kalali

Maniphest Tasks: T28819

Differential Revision: https://phabricator.mitk.org/D586