Tentative reworking of GettingStarted

See this thread for background:
https://groups.google.com/d/topic/tiddlywiki/sDWP4NPe8XE/discussion

Translators: please don’t translate this material until it’s settled
down
Fix checking for non-empty folders in initcommand
Disable links when transcluding caption field in toc macro

@BramChen does this work for you?
Typo
Adjust field and has filter handling of missing tiddlers

Now `[!field:name[value]]` and `[!has[name]` will treat missing
tiddlers as not having the specified field
Merge remote-tracking branch 'upstream/master' into fred
Remove dependency on overriding the core toc macro

Thanks to 5b38c21a417d2e5e2b85aed8010c88af32420e24
and a subsequent suggestion from @BramChen