tmf: Add a handler for UI refresh operations

tmf: Add a handler for UI refresh operations

To avoid having many many threads calling Display.asyncExec()
concurrently, offer a way to "coalesce" those UI operations.

Components can now call TmfUiRefreshHandler.getInstance().queueUpdate()
instead, which will accumulate updates coming from many components,
keep only the latest one from each component, and send them all to
the UI thread in one go after a timer has expired.

Change-Id: I367a59584a9a2b57d8409df603fc120bea993cd0
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/24479
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Hudson CI