[Livehttpheaders] Bugfix for livehttpheaders on FF/Mac

[Igor Clukas]

Hi live http headers team,

please change this single code line in TasksOverlay.xul from

  <key id="key_viewLiveHTTPSidebar" key="L" command="viewLiveHTTPSidebar"
       modifiers="alt"/>

to

  <key id="key_viewLiveHTTPSidebar" key="L" command="viewLiveHTTPSidebar"
       modifiers="accel"/>


so that Mac users can enter the @-Sign again (Alt-L on Mac). PC behaviour
won't be changed by this.

Thanks,
Igor.