gsoc:central list of columns, added sync and remove column list on db_structure page

Signed-off-by: Smita Kumari <kumarismita62@gmail.com>

fixing indentation in mult_submits.inc.php

Signed-off-by: Smita Kumari <kumarismita62@gmail.com>

naming convention,typos, indentaion fix and removed unnecessory file db_sync_central_columns.php

Signed-off-by: Smita Kumari <kumarismita62@gmail.com>

check if central columns storage is configured, if not configured don't show the option to sync/remove

Signed-off-by: Smita Kumari <kumarismita62@gmail.com>

add/remove columns to central list of columns from tbl_structure page

Signed-off-by: Smita Kumari <kumarismita62@gmail.com>

added icon image for central columns add/delete

Signed-off-by: Smita Kumari <kumarismita62@gmail.com>

add/remove of columns option in tbl_structure page in columns rows depending upon if column is already present in central columns or not

Signed-off-by: Smita Kumari <kumarismita62@gmail.com>