Introduce ImageProvider class

[hebasto]

The new ImageProvider class allows images in QML to be loaded using QPixmaps.

The last commit is a demo for our fellow designers.

For testing this PR one can run bitcoin-qt with different values of the -chain=... command-line option.

[promag]

Concept ACK. This is the correct way of exposing the dynamically generated icon based on the chain.

However, I wonder if we want to instead ship with an SVG for each chain?

[hebasto]

However, I wonder if we want to instead ship with an SVG for each chain?

Does using SVG make ImageProvider unneeded? How about caching?

[promag]

Something like Image { source: "qrc:/mainnet.svg" } should work without custom image provider. What cache?

[promag]

Let's discuss whether we want to use SVG. Qt support is detailed at https://doc.qt.io/qt-5/svgrendering.html.

[hebasto]

Something like Image { source: "qrc:/mainnet.svg" } should work without custom image provider.

It should work for any image, even non-svg.

What cache?

https://doc.qt.io/qt-5.12/qquickimageprovider.html#image-caching

[jarolrod]

@Bosch-0 has an opinion on the usage of SVG, per Icon Policy Documentation:

"Both an icon source file, in Scalable Vector Graphics (SVG) format,
and an optimized production file, in Portable Network Graphics (PNG) format"

"SVGs are used as source files because they can scale while retaining image quality."

[hebasto]

@Bosch-0 has an opinion on the usage of SVG, per Icon Policy Documentation:

"Both an icon source file, in Scalable Vector Graphics (SVG) format,
and an optimized production file, in Portable Network Graphics (PNG) format"

"SVGs are used as source files because they can scale while retaining image quality."

Being based on OpenGL and hardware acceleration, the Qt Quick framework could have a native support for SVG images without any performance penalties. Unfortunately, I have no enough expertise in this area for now.

The only drawback is introducing a new Qt SVG module.

[hebasto]

Rebased after the recent sync.

[promag]

Code review ACK b7171c8, tested but in a different branch.