Rework inheritance structure

All tests are passing, there are no functional changes.

Notes:
* created handler methods, allowing #clear and #search to be independent of the dom; avoids having to re-define 'click .close' in ClientSideFilter
* moved _debounceMethods into AbstractFilter; @wyuenho any reason why #clear and #search shouldn't be debounced in all filters (not just ClientSide)?

Refs https://github.com/wyuenho/backgrid-filter/issues/3