This restores the behavior on KWin4: if I set an X property that doesn't have any data on a window, it's still information, so this makes the Xcb wrapper return an empty QByteArray that is not null.
EffectWindow::readProperty() now returns an empty QByteArray constructed the same way as it was in KWin4.
REVIEW:118645
BUG:335446
The Xcb::Property can wrap the xcb_get_property call and provides
convenient access methods to read the value of the reply with checks
applied. For this it provides a templated ::value method for reading a
single value or reading an array. There's also a ::toBool and
::toByteArray which performs the conversion directly with default values
for the type and format checks.
Xcb::TransientFor is changed to be derived from Property instead of
Wrapper directly, so that the reading of the property value can be
shared.
Xcb::StringProperty is a convenient wrapper derived from Property to
handle the reading of a string property providing a cast to QByteArray
operator. This replaces the ::getStringProperty from utils. Though the
separator functionality from ::getStringProperty is not provided as that
is only used in one function and handled there.
All the custom usages of xcb_get_property or getStringProperty are
replaced to use this new wrapper. That simplifies the code and ensures
that all properties are read in the same way.
REVIEW: 117574
Marco Martin