(PhMediaPanel) Use PhTime instead of PhFrame

This aims at making Joker multi-speed compatible.
PhFrame is dependant on the timecode type, which may cause problems
in a multi-speed context. PhTime is safer.

To achieve this, a couple of interfaces have been added in
PhVideoEngine, PhTimeCodeDialog, PhTimeCodeEdit and PhClock to avoid
manipulating frames and work on time.
Use goToNextFrame and goToPreviousFrame
Convert timestamp change from PhFrame to PhTime

PhTime is safer than PhFrame in a multi-speed context
Remove one setFrame in PhVideoEngine

This particular use of setFrame was not dangerous, but in general
setTime is prefered because it is safer in a multispeed context.
Replace uses of PhFrame in Change_timestamp_triggered

PhTime is safer than PhFrame in a multispeed context
Replace use of setFrame in actionTimecode_triggered

setFrame is dangerous in a mutli-speed context, setTime is safe.
Remove use of PhFrame in Synchronizer

Clocks may have different timecodes, so it's safer to use PhTime
instead of PhFrame here.
Replace use of setFrame in PhLtcReader

Although this particular use of setFrame is ok, setTime is generally
safer in a multispeed context.
Remove use of setFrame from PhSonyMaster(Slave)Controller

Although this particular use of setFrame is ok, setFrame is not safe
in a multispeed context and setTime is preffered.
Comment the fact that setTime is preferred over setFrame
Remove dangerous PhVideoEngine::setFirstFrame

Setting a frame number through an interface is dangerous in a multispeed
context. Use a time instead.
Use time instead of frame in PhTimeCodeDialog/Edit interfaces

Also add a lastFrameTime() method in PhVideoEngine, and fix tests
following the modifications in PhTimeCodeEdit.

With these changes, the dialog is less exposed to errors in a
multi-speed context.
Set the media panel properties after setting up the video engine

Otherwise, when opening a video with a fps different from the
previous one, the media panel scrollbar would not be set up
properly.
Merge pull request #230 from tlecomte/tlMultiSpeed

Tl multi speed