Clean up the code to build successfully on macOS

This commit does some code cleanup to successfully build dali-core in
macOS using the Apple provided clang compiler.

The clang version shipped with macOS doesn't have the
-Wno-class-memaccess and the -Wno-cast-function-type switches, so we
test their existence before adding them to the compiler options.

Mutex now has move constructor and assigment operator, so classes with
mutexes members can use their defaults move operations. Classes holding
references cannot have a default move assigment operator.

Virtual functions overriding parent classes are now declared with the
`override` keyword.

MAXIMUM_TIME_DIFF_ALLOWED is a global internal variable nerver used, so
it was removed.

.gitignore was updated to include some artifacts from coc-nvim
extension.

Change-Id: I76201f867240a0822e7e202c2edcbfa07b2dffd6