Minor XUL corrections and warning for missing translations

Fixes positioning problem with xul:splitter due to disregarding offsetParent.

Fixes flex problem with xul:tabbox due to missing viewType specification (issue #175).

Fixes manually entered value problem with xul:timepicker (issue #178).

Fixes xul:textbox ignoring width and height attributes due to missing "px" in concatenation (XUL specs do not allow units in width and height attributes).

When a translation for a locale key cannot be found—neither in the selected locale nor in the default one—and when console.warn is defined, then a warning message is generated.