Fix find_next_section behaviour in complex tree scenario.

[kabalin]

This patch makes find_next_section respect cases where requested section contains subsections, it fixes:

• Do not return null if requested section contains subsections.
• Make it identify the last section correctly.

Scenarios to test this patch:

1. Setup sections as:

   section1
   section2
   section3
       section4
   section5 (add activity to observe where section is moved)
   section6
   section7
   section8
   section9

2. From scenario at step 1 move section5 before section 7 (before this patch section5 will become the last section8)
3. From scenario at step 1 move section6 before section 5 (before this patch section6 will not move)

Fixes #37 (hopefully)