1. freeze row (or columns) of each sheet independently with API by
passing row index (or column index).
2. unfreeze rows (or columns) of each sheet independently with API by
passing -1.
For new data structure of freezing rows (columns), re-implement spec
about 
* inserting and removing rows (columns)
* max visible rows changes
change implementation. We store frozen rows (columns) information of
each sheet in Spreadsheet component instead of client-side. Hence, we
ignore data from client.
add test app for ZSS-206
refine test app for ZSS-206
* remove unused variables about freezing
* refine Javadoc
* add test cases steps
* ZSS-399: add test case
* update release-note
* add zsspdf and zsshtml Maven dependencies into zss.test
* turn on ZK javascript debug mode
ZSS-420: 
 * fix freeze panel index misunderstanding issue.
 * add test case.
 * update release-note.
ZSS-421:
 * fixed: calculate in-line editor must consider the freeze panel
 * add test case
 * update release-note
ZSS-206	Row/Column freezing remains for a sheet until you freeze
rows/columns on another sheet.
* refine comments
ZPOI 3.9: Migrate to 3.9, update zpoi version to 3.9