Use the word "symbol" instead of "tag" in the UI and documentation

Use the word "symbol" instead of "tag" in the UI and documentation

For users a tag is <this> so the naming can be confusing.

The only exception where we probably shouldn't use the word symbol is the
"tags file" (*.tags) containing global tags - this has already the "tags"
extension and is more related to ctags and using "symbols file" is a bit
strange in this case.

As a result, the only places where this patch leaves the word "tag" are:

* phrase "tags file(s)"
* phrase "tags parser(s)"
* documentation mentioning the "tags" directory
* documentation mentioning the *.tags extension

and of course where it means the HTML/XML markup <thing>. The rest of the
uses of the word "tag" is replaced with "symbol".

Documentation is updated accordingly.

Fixes #579.