Tabbed forms shouldn't show tabs with no visible fields

[scottnash]

Since some fields on a component form are only shown conditionally, you could theoretically hide all fields on a tab, but currently the tab still shows when all the fields on it are hidden. Also there should be a shortcut in kiln.js to hide a tab, which would hide all the fields on that tab, and then when the tabs are behaving correctly (i.e. not appearing when no visible fields are on it) this would hide the tab.