Initial Archive Handlers visual implementation

The Archive Handlers dialog skeleton now implemented - doesnt do
anything, but demonstrates I can implement the mockup
Archive Handlers visual implementation v2

Code style changes, dialog expansion fixed, terminal checkbuttons
implemented for commands
Removal of previous Archive Handler implementation now its been
approved
Archive Handlers visual implementation v3

Rename General to 'Handler Settings', rename appropriate widgets
Change code style to standard C
Removal of suspicious underscores in default archive type names
Code style - replaced tabs with spaces
Beginning of logic behind GUI

Committing WIP changes from a few months back in preparation for the
current programming stint
Archive Handlers GUI logic 'complete'

Handler changing, addition, deletion, restore to defaults
Always cast the result of g_object_get_data to the proper type
Make handlers list reorderable and persist changes
static for all private functions
Prevent spaces in handler MIME data
ptk_file_archiver_is_format_supported reimplemented
ptk_file_archiver_create and on_format_changed reimplemented
Deal with empty commands yet "run in terminal" ticked
Revert spurious change in ed2d027c
Add validation to enforce placeholders in commands
Archive creation now uses placeholders
ptk_file_archiver_extract reimplemented, do not mandate "%f" on archive extraction