Qt 5.4 again

[Jonathan Riddell]

Last we looked we decided that Plasma 5.2 can depend on Qt 5.4.  But
come beta time it seems nobody has felt the need to change any modules
to require on Qt 5.4.  Martin G says KWin has a bunch of fixes that
work with 5.4 only but it'll also work fine with Qt 5.3.  The KDE CI
system has Qt 5.3 so updating a build requirement would make that
break.

So I plan to announce Plasma 5.2 as working with either Qt 5.3 or 5.4
but it'll have some fixes if you use Qt 5.4.  I'll also say these are
build time fixes so packages should be recompiled after updating a
distro to Qt 5.4.

Comments welcome

Jonathan