This commit adds the mathed command \mathds that selects a
font more appropriate than \mathbb for typesetting the
mathematical symbols for the natural (N), whole numbers (Z),
rational numbers (Q), real numbers (R), complex numbers (C),
and some others.
As in the \mathbb case, only capital letters are supported,
but in addition one can also typeset a symbol often used for
representing the indicator function (\mathds{1}) and the
letters a, h, k.
Fixes bug #11887.
There is a chance that this code is still useful for Qt 4. We keep
it around just in case.
For the case of Qt 4, this commit restores the code changed in
f8e40f8a, d255339a, and d4ff3a29.
Setting the family to "family" is already attempted above. A
different case was adapted to be this duplicate case at d4ff3a29,
but it should have just been removed.
Spotted by Enrico.
In particular, the directory frontends/qt4 is renamed to frontends/qt.
Many configurations file have to be updated. All mentions of qt4 in
the source have been audited, and changed to qt if necessary.
The only part that has not been updated is the CMake build system.
