Add ELEMHIDE to AdblockPlus::FilterEngine::ContentType enum

[sergz]

It's needed by IE addon otherwise it's not compilable. In the recent libadblockplus FilterEngine::Matches accepts only enum AdblockPlus::FilterEngine::ContentType, it does not accept string as before.
We should fix the exactly enum for consistency with other filter options. The another option is to add a special method for elemhide case and refactor (add) FilterEngine::CheckFilterMatch which accepts content type as string and we can call it from Main.cpp. It might be even better, because I'm not sure whether it makes sense to call CheckFilterMatch(url, 'document', url) for the case.

What to change

Add the enum element and fix ContentTypeMap.

[fhd]

Yeah, lets'add ELEMHIDE to the enum. While we're at it: POPUP is also missing.

[philll]

And what could be tested here? :-)

[sergz]

Replying to fhd:

Yeah, lets'add ELEMHIDE to the enum. While we're at it: POPUP is also missing.

POPUP is not mentioned in ​https://adblockplus.org/en/filters although I see it in the filters. BTW, it's not used in IE. I would say we can add it in another commit.

[sergz]

Replying to philll:

And what could be tested here? :-)

I'm not sure whether it makes sense to write any test now because we don't have such tests at all, which verify whether each filter option is supported and works perfectly. Although I would certainly like to have automatic tests. Currently I would classify the issue as some kind of typo.

The real trouble it causes is that it's not possible to use the recent version of libadblockplus in IE project without that enum entry. Of course we test that it works during debugging, despite it's not on what we should rely. May be testers can find some page which has elemhide filter and test whether ABP properly works but firstly we need to update the library client, for example IE addon.

[trev]

Replying to philll:

And what could be tested here? :-)

Nothing - this is a libadblockplus issue. Testing can be done once these changes are "imported" into an actual product that uses libadblockplus, like Adblock Plus for IE.