Add option to process GPX extensions nodes

Fixes #1791.
Add tests for GPX extensions
Automatically dispose the ViewportSizeMonitor
Dispose the tiles removed from the cache
Merge pull request #2420 from gingerik/gpx-extensions

PR for GPX: read extensions tags
Unconditionally add version info to the build header

This reworks the changes in #2442 so we always include the version info, regardless of whether the module is run as main or not.  This also addresses a few lint related issues.
Merge pull request #2446 from tschaub/version

Unconditionally add version info to the build header.
Fix WKT writers output type
Improve wording of ol.Map description
Merge pull request #2449 from probins/patch-1

Improve wording of ol.Map description
Fix typo in ol.Map test.
Merge pull request #2452 from marcjansen/test-typo

Fix typo in ol.Map test.
Remove reference to simple/whitespace in examples/readme
Merge pull request #2457 from probins/patch-1

Remove reference to simple/whitespace in examples/readme
Merge pull request #2423 from fredj/dispose

Dispose the tiles removed from the cache
Merge pull request #2448 from gingerik/wkt-docs

Fix WKT writers output type.
Remove function bodies from externs
Add ol.source.Cluster

Initial code by @kenny806
Update link to closure-library issue
Merge pull request #2464 from fredj/closure-library-issue-link

Update link to closure-library issue