Merge headers when a single section of search results is displayed

Merge headers when a single section of search results is displayed

There is no need to show two headers when we are only displaying a single
section of search results, so we merge the section header with the main
header in that case.

Show a back button on the left of the main header to get back to the results
for all sections.

Remove this._showTooltip flag from SearchSectionHeader because we no longer
show this type of header when we want to suppress the tooltip.

Add this._appSearchResultsOnlyShown and this._docSearchResultsOnlyShown
flags to track when a single section is shown more cleanly.