Add support for task "grouping"

[rshk]

Tree structure VS tags + "batch build" button

• with a tree, we can better unclutter the UI using a "folder-like" structure
• with tags, we can have multiple categorizations

Example categorizations:

• Crawl FOO -> harvester, crawl, foo
• Crawl BAR -> harvester, crawl, bar
• Convert FOO -> harvester, convert, foo
• Convert BAR -> harvester, convert, bar

BUT the above tasks are all part of the "harvester" group

[rshk]

duplicates #10