#9 v7.7 fixes for Mac/Qt 5.6 builds

[R.J.V. Bertin]

I've been spending some time creating an updated MacPorts port for kchmviewer 7.7, building against Qt 5.6.2 . This required me to apply a certain number of patches, partly to point the build process to the appropriate locations and to accept QtWebKit 5.6 (which works just fine).

I'd like to contribute those modifications that are of general interest:

• in src.pro, use ICON=foo.icns instead of a custom command via QMAKE_POST_LINK (that no longer works)
• in kchmviewerapp.cpp, use a public member function instead of a private one
• in main.cpp, use a Mac platform token that still exists so that opening documents through LaunchServices actually works
• in ebook_epub.cpp, don't abort ("crash") when trying to change the encoding (it would be better to disable the whole menu, of course).

We have a functional DBus infrastructure in MacPorts, so I reenabled DBus support in our Mac build. I'm not yet sure if there's a point in that.