Dot11: use ConditionalField() to replace .is_applicable() methods

Some fields in Dot11 protocols were using specific .is_applicable()
methods to implement similar features than ConditionalField() (which
probably didn't exist by the time Dot11 dissectors have been written).

This commit uses ConditionalField and removes specific field classes
to use generic (MACField and LEShortField) fields. This should make
the code easier to understand.

This will also prevent some fields from being displayed when they
should not:

    >>> Dot11().show()
    ###[ 802.11 ]###
      subtype= 0
      type= Management
      proto= 0
      FCfield=
      ID= 0
      addr1= 00:00:00:00:00:00
      addr2= 00:00:00:00:00:00
      addr3= 00:00:00:00:00:00
      SC= 0

    >>> Dot11(type=2, FCfield=3).show()
    ###[ 802.11 ]###
      subtype= 0
      type= Data
      proto= 0
      FCfield= to-DS+from-DS
      ID= 0
      addr1= 00:00:00:00:00:00
      addr2= 00:00:00:00:00:00
      addr3= 00:00:00:00:00:00
      SC= 0
      addr4= 00:00:00:00:00:00

    >>> Dot11(type=1).show()
    ###[ 802.11 ]###
      subtype= 0
      type= Control
      proto= 0
      FCfield=
      ID= 0
      addr1= 00:00:00:00:00:00