Allow installation of extra platform plugins

[asl]

Resolves #88

[asl]

This is essentially code from #89 but refactored to address review requests.

I checked that indeed it does deploy extra platform plugins and the final AppImage works

[asl]

Tagging @TheAssassin

[bjorn]

I'm just wondering, but why do we need EXTRA_PLATFORM_PLUGINS in addition to EXTRA_QT_PLUGINS? Are platform plugins not Qt plugins as well?

[asl]

@bjorn Apparently, yes. EXTRA_QT_PLUGINS is a pre-defined hardcoded list:

linuxdeploy-plugin-qt/src/qt-modules.h

Line 21 in dc2e5b7

static const std::vector<QtModule> Qt5Modules = {

Actually EXTRA_QT_PLUGINS name is a bit misleading, it should actually be EXTRA_QT_MODULES.

[asl]

@TheAssassin will you be able to review this? Thanks!

[asl]

@TheAssassin gently ping

[TheAssassin]

Actually EXTRA_QT_PLUGINS name is a bit misleading, it should actually be EXTRA_QT_MODULES.

I think this was just taken over from linuxdeployqt. We should think about correcting this. Mind to open an issue?

[TheAssassin]

Haven't tested this extensively, but the code looks good and on a first try it seems to do the trick. Thanks @asl!