Insert a special candidate for the wildcard character itself if only a wildcard character has been typed

Resolves: rhbz#1110325 - Unable to input question marks in Wubi-jidian
See: https://bugzilla.redhat.com/show_bug.cgi?id=1110325

The same problem has been also reported here:

    https://code.google.com/p/ibus/issues/detail?id=1720

If only a wildcard character has been typed, insert a special
candidate at the first position for the wildcard character itself. For
example, if “?” is used as a wildcard character and this is the only
character typed, add a candidate ('?', '?', 0, 1000000000) in
halfwidth mode or a candidate ('?', '？', 0, 1000000000) in fullwidth
mode.  This is needed to make it possible to input the wildcard
characters themselves, if “?” acted only as a wildcard it would be
impossible to input a fullwidth question mark.
Make wildchard characters configurable

The options AUTO_WILDCARD, SINGLE_WILDCARD_CHAR, and
MULTI_WILDCARD_CHAR are now not only configurable in the table sources
but also in the setup tool and via dconf.

The default for AUTO_WILDCARD if neither specified in the table source
nor in the setup tool nor in dconf is TRUE, this is best for most
tables.

The defaults for SINGLE_WILDCARD_CHAR and MULTI_WILDCARD_CHAR
are the empty strings, this means no wildcards are used.

In some table sources default values for SINGLE_WILDCARD_CHAR and
MULTI_WILDCARD_CHAR are defined, in most cases these are

SINGLE_WILDCARD_CHAR = ?
MULTI_WILDCARD_CHAR = *
Update German translations
Update Chinese .po files

(Only to get the changed strings from the source synced with the .po
files, I did neither change nor add any translations).
Merge pull request #41 from mike-fabian/improve-wildcard-support

Improve wildcard support