qt4apps: remove MarNetworkWidget
qt4apps: remove all Marx... apps

Graphing is implemented in "inspector" app using Qwt, and these apps have
no significant contribution.
qt4apps: remove MarMonitors

Comparable functionality is being implemented in the "inspector" app.
qt4apps: remove MarWidgets

It was incomplete, no significant value.
qt apps: separate headers common to Qt4 and Qt5 apps; remove dead code

Currently, there are no headers being installed that are related to
Marsyas-Qt integration. This may be remedied once I analyse and confirm
the new headers to be safe and useful in external use.