debian: set DEB_PYTHON_INSTALL_LAYOUT=deb to work around broken --prefix

I don't know what's changed in distutils/setuptools, but --prefix=/usr is
no longer respected without --install-layout=deb.

htmlui: don't pass bytes to extensions.wrapfunction()

hg 6.6 no longer accepts bytes, and older hg versions appear to be okay with
unicode function name.

app: regenerate ui code by qmluic d60dd06cb808

hgchangesetpanel: add user command actions to widget so shortcuts are handled

hgchangesetpanel: remove unused includes

mainwindow: add action to close (or unsplit) current text panel

mainwindow: add helper to find next in circular list

mainwindow: remove useless default from collectSplitterChildren()

mainwindow: on select other panel, look up ancestor panel widget first

debian: adjust debian-qt6 rules for Qt 6 build

debian: copy debian-qt5 to debian-qt6 for modification

make: require QT_VERSION_MAJOR to select debian packaging data

debian: enforce QT_VERSION_MAJOR

debian: bump required Qt version to 5.15

debian: include README.md in package

debian: reformat control file

debian: update copyright file

debian: remove useless README file

make: backport debian packaging rules from qt-emacs-key-theme

debian: remove stub changelog