[FEAT] Allow zoom in and zoom out through a slider widget

[NathalieC]

Prerequisites: #1487 #1488

Multiple clicks on "+" or "-" buttons (#589) could allow for zoom in and out, but it may take a lot of clicks to get to the comfortable zoom level.

So, in the application embedding the library, a slider could be provided: the user drags and drops the cursor.
The appplication that embeds the library passes a parameter to the library (zoom ratio? absolute value?) and the zoom level adjusts accordingly.

[tbouffard]

From the library point of view, covering this use case means

• provide a function/command to set the zoom level [FEAT] Add API to zoom programmatically #1487
• a zoom can be triggered in other ways (for instance with mouse wheel, zoom reset, fit), the lib must provide a way to notify application/client of zoom level changes. This could be managed by events, listener, or any mechanism or allow to bind the internal zoom factor to a variable hold by the application. See [FEAT] Send events on Zoom #1488
• if min/max zoom level is supported ([FEAT] Set zoom in / zoom out max values #732 and [FEAT] Provide visual/auditory clues when zoom max values are reached #731), see how this feature articulates with zoom constraints