Review user interaction in session management

Summary:
* SessionManageDialog
    - Build user interface by ui file
    - Avoid obsolete Qt members
    - Choose more meaningful member names
    - Add filter field and sort button
    - Open session by double click
    - Add buttons for "Copy" and "Open as Template"
    - Reorder a couple of code to be a little bit more logic ordered
    - Delete a session with delay which offer a restore and avoid annoying
      confirmation dialog
    - Rename a session inside of the list view to avoid extra popup
      window to enter the new name.

* Remove SessionOpenDialog, use SessionManageDialog instead
* Remove SessionChooser, use SessionManageDialog instead

* SessionManager
    - Add signal sessionListChanged()
      To avoid unneded signals is updateSessionList() slightly modified
      with a clearer look and an added check for changes in an easy way.
    - Add copySession()
    - Let rename/copySession() ask for a new name when needed
    - Move session creation parts from newSessionName() to sessionSaveAs()
    - Rename newSessionName() to askForNewSessionName()
    - Add suggestNewSessionName()
    - Don't create anonymous session in ctor
    - Don't save anonymous session as last session

* MainWindow
    - Remove from sessions menu "Open Session" because it's now the
      same as "Manage Sessions"

Test Plan:
{F6443763}
{F6443766}

Reviewers: #kate, dhaumann, ngraham, #vdg, cullmann

Reviewed By: #kate, dhaumann, cullmann

Subscribers: fabianr, anthonyfieroni, cullmann, kwrite-devel, #kate

Tags: #kate

Differential Revision: https://phabricator.kde.org/D16926