has-tabs refactor

[adamdbradley]

• Change has-tabs to has-footer-tabs
• Add has-header-tabs
• If has-header-tabs/has-footer-tabs is set, then use their values
• If neither has-header-tabs/has-footer-tabs is set, then automatically figure it out
• Add has-platform-tabs which will automatically figure out if the tabs should be in the header (Android) or the footer (iOS)

[ajoslin]

related: #619, #395, #419

[perrygovier]

Moving feature requests from GH Issues to Trello. Please continue the discussion and vote for this and other issues that are important to you here:
https://trello.com/c/ACyoceaf/17-has-tabs-refactor

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