Removed private methods of plugins and generalized plugins' access

[marizuccara]

Short description of changes:

Removed all private methods of plugin classes, so the plugins can be extended avoiding duplicated code, because private methods can not be inherited.

Allow to extend also the Region class, introducing the possibility to create your RegionCustom class that extends the basic one, overriding its methods if necessary.

Generalized access to plugins object: in some plugin classes, the access to their own params was made from the outside. For example, to access to the timeline plugin duration parameter, the access was done in this way:

this.wavesurfer.timeline.params.duration
instead of:
this.params.duration.

Accessing this parameter from the outside made it impossible to call custom plugins with custom names, because to the wavesurfer object is assigned a plugin with the name you give it. So, if you for example call it timelineCustom, the access to the duration parameter from the outside, should be instead this.wavesurfer.timelineCustom.params.duration. But in this way you cannot generalize the plugin class.

Breaking changes in the internal API:

• Removed private methods from the cursor.js and timeline.js files.

• In the timeline plugin, access to the duration parameter from its params object.

• Added export clause to the Region class, so it can be also extended.

• Access to the getRegionSnapToGridValue method of the regions plugin from the internal util object, instead of from the outside.

• Pass to the Region class constructor, the util object of the RegionsPlugin class, so the getRegionSnapToGridValue method have not be accessed from outside.

Related Issues and other PRs:

Issue #1914
PR #1921

[coveralls]

Coverage remained the same at 80.62% when pulling 7785908 on marizuccara:no-private-plugins into d8f5c32 on katspaugh:master.

[marizuccara]

Another thing. The Region class can be moved to a separate file? This is because if we export both Region and RegionsPlugin class, we have errors when trying to create the plugin in this way:

Wavesurfer['regions'].create(options)

bacause `Wavesurfer['regions']' give this:

so create method cannot be accessed directly, as when creating the plugin in this way:

RegionsPlugin.create(options)

[thijstriemstra]

Another thing. The Region class can be moved to a separate file?

If we do that I would place the plugin files in it's own directory, e.g. src/plugin/regions/. This has implications for the build process but shouldn't be too hard to fix.

Splitting into multiple files/classes would be useful for the other larger plugins as well.

And can you also add a changelog entry?