Examples: Add Q_DECL_OVERRIDE to overridden functions

Q_DECL_OVERRIDE (which expands to 'override' for supported compiler)
helps to declare the intent (that it is an overridden function) and
force compilation error when there is no such virtual function in the
base class.
The examples should show the best practice of having it, as it may save
the programmer quite some time in case of change of API or typo in the
function name or arguments.

This change was done automatically with
clang-modernize -add-override -override-macros

And fixed MSVC compilation by removing inline for TorrentViewDelegate::paint

Change-Id: Ice66ae93fae571266f908703d5b8892b2c1ebb1a
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>