Remove platform-dependent code from ClipboardMonitor

The platform-dependent code for handling clipboard needs to be
reimplemented in appropriate platform subdirectory.
Build fixes for cmake and qmake
Windows: Use native functions to check clipboard
Use system-wide mutex when using log for a session
Build fixes for X11 and OS X
Re-create session mutex when starting server or client

If server crashes, the system mutex is not released on Unix systems. It
is needed to explicitly state when to create the mutex.
X11: Postpone selection changed signal until it's complete
X11: Reset and synchronize clipboard and selection
Use clipboard mode in plaftorm interface
Fixed documentation for PlatformClipboard::loadSettings()
Build fix and MIME_PREFIX macro rename
X11: Synchronize immediately is clipboard is being set
Use session name for mutex from COPYQ_SESSION_NAME
Create session mutex after server is created
OS X: implement MacClipboard.
Merge pull request #257 from rygwdn/monitor_rewrite

OS X: implement MacClipboard.
Move tooltip to correct widget in action dialog
Merge branch 'monitor_rewrite'