MainWindow: only trigger link tooltips of Mumble is the frontmost program.

Fixes #1372
Fix hide in tray default value for Windows 8.1

Fixes #1126
Introduce use of Q_DECL_OVERRIDE and Q_DECL_FINAL into the codebase.

Edited header files and tried to mark all overriden methods with
Q_DECL_OVERRIDE. Also added Q_DECL_FINAL in a few places where
a non-virtual destructor was used in a class.

Q_DECL_OVERRIDE/Q_DECL_FINAL were introduce with Qt 5.0. For earlier
versions Q_DECL_FINAL is defined to nothing. Q_DECL_OVERRIDE is still
available on windows as the override keyword was supported there since
msvc 2010.

All new code should make use of these macros as the bugs that occurred
in the Qt 4 to 5 transition show they are needed.
Replace overlooked QT_VERSION_CHECKs with explicit version.

See 12eb26431f03f5ed3c634d647b1581572039112d .
Fix hide in tray on windows.

Another override that did no longer apply after the
Qt 5 transition. In this case winEvent which we
overrode in Qt 4 was replaced with nativeEvent in
Qt 5.

Partially fixes #1336
Fix ServerView::mimeData not actually overriding base class function.

Fixes drag & drop behavior of ConnectDialog. Previously dragging an
item out of the connect dialog didn't yield usable data. With the
override working we get the URL as expected.
Replace hacky hide/show handling with more straight-forward implementation.

Fixes #1336
Special case the Unity desktop to also default to not minimize to tray