Rename QSystemLocale::fallbackLocale() to QSystemLocale::fallbackUiLocale()

Rename QSystemLocale::fallbackLocale() to QSystemLocale::fallbackUiLocale()

Suggested by Oswald Buddenhagen. This function is about the (main)
UI language, not about other locale features like number formatting etc.
It not in the public API anymore in Qt 5.0 so it can be renamed.

Change-Id: I2a23892c67e5813de4c0e57330749568777e9ee6
Reviewed-by: Oswald Buddenhagen
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>