break down the katession.h/.cpp files

Changes:

One class per file. Code is now testable (almost) independently from
the app, so adding bunch of unit tests.

KateSession
* simple container
* remove read/writeConfig() in favour of config()
* move rename method to manager
* decouple name from filename (manager does the coupling now)
* hide constructor in favour of factories returning KateSession::Ptr
* not dependent on KateSessionManager anymore
* add sorting functions
* add timestamp (last time the session was saved)

KateSessionManager
* add activeSession(QString, bool, bool, bool)
* add activeAnonymousSession()
* change internal storage from QList to QHash (faster lookups)
* remove sorting from sessionList() to callers
* use KDirWatch to monitor session dir (this removes the constant
updates of m_sessions)

KateSessionsAction
* add possibility to depency-inject the KateSessionManager
* sort by modification time (desc)
* limit to ten items
* disable action, when there are no sessions

KateSessionChooser
* guarantee that selectedSession() will be valid, when result is resultOpen or
resultCopy. This simplifies the KateSessionManager workflow.



FIXED-IN: 5.0