Make Icons available through QML Icon Themes

[jarolrod]

Currently, with the way our Image Provider is set up, every Icon will need an entry under
1.Makefile.qt.include

QML_RES_ICONS = \
  qml/res/icons/sun.png\
  ...

2.bitcoin_qml.qrc

 <qresource prefix="/icons">
        ...
        <file alias="sun">res/icons/sun.png</file>

3.imageprovider.cpp

if (id == "sun") {
        *size = requested_size;
        return QIcon(":/icons/sun").pixmap(requested_size);
}

Then access to the Icon will require explicitly stating its URL, for example: icon.source: "image://images/sun"

We should instead make use of QML's Icon Themes. Instead of maintaining all these different entries in order to serve one icon, we could have an index.theme file which lists all entries. Then Icons are available by name, for example: icon.name: "sun"

Useful skills:

Qt5/6, QML

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[shaavan]

Let me try giving it a shot!

[jarolrod]

@promag do you think this is better than the imageprovider we use?

[jarolrod]

TODO when migration to cmake and QT6 is done