Fixes GH-3118
Fixes GH-3720
Fixes GH-3731

Icons and Ok button state will now switch consistently when moving
between tabs. ATLaunchers packlist is now no longer redownloaded
each time you open its Tab. All lists are striped now. And all
search and filter fields now have a placeholder text.

This resolves an issue with the modpacks.ch search functionality, in
which a search issued while one is currently in progress won't be made
and the UI won't allow for the search to be made after.

Reproduction Steps:
1. Open the FTB pane in the Add Instance Dialog
2. Perform a search while MMC is still performing the initial search

The search won't be performed, the existing search will have been
aborted, and you are unable to try the search again (without trying a
different search in the meantime).

This was caused by 2 things:
1. A search cannot be re-attempted, and this logic doesn't consider
   failures.
2. The failure slot wasn't called when the NetJob was aborted, so
   the search would never be performed - but the term would be
   stored as if it had (trigering point 1).

I have resolved this by doing 2 things:
1. If the failure slot is called, set a searchState of Failed. Allow
   search re-attempts in this case.
2. If there is a present NetJob, abort and reset it. The immediately
   continue with the search.

This was a mishap on my part, fortunately quickly exposed as the
combination of GH-3463 and GH-3464 made no packs visible.

NOISSUE Ignore 'dud' FTB packs

There is no guarantee from modpacks.ch that modpacks contain any
versions, which is currently an issue with pack 63 (Direwolf20 1.5).