API: Refactor buffer_{get,set}_line

They are now implemented on top of the buffer_{get,set}_slice functions
API: Implement functions for deleting lines
API: Implement buffer_{get,set}_var
API: Implement buffer_{get,set}_option
API: Implement buffer_{get,set}_name
API: Implement `buffer_insert`
API: Implement `buffer_is_valid`
API: Implement `buffer_get_mark`
Add new files to clint and fix reported errors
API: Implement `window_get_buffer`
API: Implement window_{get,set}_cursor
API: Implement window_{get,set}_{height,width}
API: Implement window_{get,set}_var
API: Implement window_{get,set}_option
API: Implement window_get_position
API: Implement window_is_valid
API: Implement tabpage_{get,set}_var
API: Implement tabpage_get_window
API: Implement tabpage_is_valid
API: Implement tabpage_get_window_count