* utils/var_bool.*: VarBool is now an interface for reading bool variables

* utils/var_bool.*: VarBool is now an interface for reading bool variables
 (not writing); use VarBoolImpl instead to instanciate read/write variables.
 Bool variables can now be combined with VarNotBool and VarBoolAndBool
 (TODO: VarBoolOrBool)
* commands/cmd_show_window.hpp: the commands now call directly
  GenericWindow::show/hide, because the visibility variable of a
  window is a VarBool (so, read-only)
* commands/cmd_input.hpp: added Play and Pause commands
* parser/interpreter.cpp: beginning of support of boolean expressions,
 like "vlc.isSeekable and not vlc.isStopped" (operator precedence is
 not really well handled yet)
* src/vlcproc.*: new variables "vlc.isSeekable", "vlc.isStopped" and
 "vlc.isPaused"
* controls/ctrl_checkbox.cpp: the "state" variable of a checkbox is
  now a passive VarBool, so actions must be explicitely set with
  'action1="..." action2="..."' in the xml file
* removed src/vlcvars.* => "vlc.isMute" doesn't work any more
 (anyway it didn't work well...)
* theme/theme.xml: updated with the new VarBool behaviour