Show debug button when KDevelop session is running

Summary:
- independently of the config setting in drkonqirc propose
  debugging with KDevelop if a session is available
- default debugger is only displayed if ShowDebugButton=true
  in [DrKonqi]
- button is the first action button to be displayed
- fix adding/removing of the button to the button box

Test Plan:
- set ShowDebugButton=false, have no KDevelop session running
- force a crash (using src/tests/crashtest for instance)
- DrKonqi shows no debug button
- start a KDevelop session
- the button appears with the KDevelop session selectable
- quit DrKonqi, set ShowDebugButton=true
- crash again, the button now always is visible, including
  the default debugger as a selection

Reviewers: #plasma_workspaces, apol, mwolff, #kdevelop, sitter

Reviewed By: sitter

Subscribers: kossebau, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11234