Ticket #3228 (select/unselect files with the same extension as the current file)

Add Action 'SelectExt' for select/unselect files with the same extension as the current file.

Signed-off-by: Ilia Maslakov <il.smind@gmail.com>
Merge branch '3228_select_files_by_extension'

* 3228_select_files_by_extension:
  Ticket #3228 (select/unselect files with the same extension as the current file)