Add: ThumbnailZoomPlusOptions check options compatibility during init

New method _checkPrefsCompatibility() checks minVersion, maxVersion,
targetApplication and targetPlatform on per element basis.
Change: RMB option for activatekey pref is version and platform dependant

The "thumbnailzoomplus-options-activate-key-pref" preference element will
be disabled unless run on:
 - WINNT or Linux
 - Firefox 15.0 or later