Make Qt apps look native on GTK systems

[probonopd]

According to https://bbs.archlinux.org/viewtopic.php?pid=1637449#p1637449

Platform theme is responsible for fonts, icon theme, default style, native dialogs and other settings. Style is responsible only for widgets rendering.
Qt 5.6 had gtk2 platform theme (which used gtk2 settings) and gtk2 style (which rendered widgets using gtk2). Qt 5.7 has gtk3 platform theme (which uses gtk3 settings), but no gtk3 style.

plugins/platformthemes/libqgtk2.so looks pretty native on Ubuntu, only the global menu bar is missing.

Also see NixOS/nixpkgs#15926.

In Qt 5.7 this seems to be broken! Supposedly it should have a libqgtk3.so but it seems to be missing entirely!

[probonopd]

According to https://bbs.archlinux.org/viewtopic.php?pid=1658017#p1658017,

They will enable GTK Theming again in QT 5.8, due november.

[probonopd]

Sometimes fonts in Qt based apps can be mixed up. Possibly

export QT_QPA_FONTDIR=/usr/share/fonts

can help (to be investigated).

[probonopd]

See https://github.com/MartinBriza/adwaita-qt to make it look native on Gtk.

[probonopd]

Seems to be a duplicate of #88, hence closing here.