Themed menus: allow Undo/Redo options to always be displayed.

This changes the treatment of the Undo and Redo menu items in the
cutlist editor so that they can always be displayed, even when no undo
or redo action is available.  By default, unavailable undo/redo
actions are hidden via the show="active" attribute.
Themed menus: Add a compact version of the cutlist menu.

The compact version is taken from menu_cutlist_compact.xml.  Revert
the addition of menu_cutlist_exit.xml, as there's really no need to
allow such customization.  (Users may want to manually remove
menu_cutlist_exit.xml from their installation directory.)  Add the
ability to take a menu description from an internal string rather than
an external file.  Refs #11533.
Remove qmake dependency on ffmpeg file that no longer exists.
Get mythbrowser to compile in Qt5 (won't do much).
Get mythnews compiling with Qt5.
Fix mythzoneminder Qt5 compile.
Export remotefile.h for plugins. Should have been exported all along.
Rename system-XXX.{h,cpp} files to use myth coding convention.
Add override checking to MythSystemPrivate inheritance.

This only works when compiling with with C++11 support, but when compiled on such
a system this will throw an error if the signature of the base class method changes
without the child class being updated.
Remove some unused members and initialise others in ScreenWizard. Silences some cppcheck warnings.
Silence a cppcheck warning about allocating m_gameTree before checking that it was deallocated.
Silence cppcheck warning about allocating m_browser before checking whether it was already allocated. False positive in that Create() should only be called once per instance of MythFlashPlayer but it doesn't hurt to code defensively against the possibility.
Remove redundant/unused assignments to chk from AudioInputOSS. Silences several cppcheck warnings
Initialise MythCoreContextPrivate::pluginmanager to NULL and add guards against leaks if SetPluginManager() is called twice
Rename pluginmanager to m_pluginmanager per coding standards
Apply the proper fix for a gcc unused variable warning, the existing fix just triggers another cppcheck warning instead.
Suppress warning about unused variable in Mythmusic's visualise.cpp
Coding standards cleanup of lcddevice.cpp/h and use initialisation list for member variables (cppcheck)
Use initialisation lists for MythEvent constructors. Cppcheck warning.
Move MythSystemPrivate out of public interface header.