Refactor Elements

[danngreen]

This PR refactors the MetaModule::Elements variant, with a few benefits:

• more simple to port VCV modules -- the last step of the Porting Guide (which was never fully documented) is now reduced to just writing a one-line function. This addresses More automation for adding a brand ported from VCV #47.
• easier to add more data to 4ms element Info structs (like switch position names)
• faster build time: 1.5x - 2x faster for firmware and simulator!
• easier to add GUI features. Examples: Firmware: Manually controlling a switch or button should be more natural #119 and Firmware: Fix display of switches #120

Previously, each widget type needed a corresponding variant alternative type in the ever-growing Elements std::variant type.
Now, the Element variant alternatives are reduced to a small set of general types (like Knob, Slider, DualLight). Each one is parameterized by non-static member vars (e.g. image, color). This makes it easier to add new types.

Other changes (besides refactoring Elements):

• Switches and slide pots can have a bg image (body) and a fg image (handle), so the rendering is closer to VCV Rack
• More Rack widget images are ported (all of them used in the current set of modules, I think)
• The new features in fix-switches-gui that make manually controlling switches/buttons "more natural" are present in this branch too
• VCV_Adaptor automatically extracts strings for switch positions from ported VCV modules, and displays them when you manually adjust a switch. This was convenient to do with the new Elements system.
• Images can be accessed by filename strings, rather than needing a pointer to the LVGL image struct. A mock read-only filesystem (image_fs.hh) translates the filename to raw data. In the future, we may use a real filesystem, so this paves the way for that.

There's almost no changes needed to 4ms modules (one very subtle difference, I'll document).

Performance-wise, I've noticed no change. I expect it to change very little, since this almost exclusively effects the GUI thread (which is bottlenecked by hardware) and pre-main startup (when Rack modules are converted to MetaModule-native structures). The binary is ~64k smaller, also.

TODO:

• Document widget porting in the Porting Guide readme.
• Fix pixel alignment issue with sliders and switches
• Allow SlideSwitches to select which direction corresponds to an increasing value
• Move std::string_view image from the derived classes to BaseElement
• Check for conflicts with other open PRs

[danngreen]

I fixed the pixel alignment issue with sliders. The cause was that VCV_adaptor just used the xy position specified in the ModuleWidget constructor calls to createParam(). Some widgets change their xy position by adding a margin, but this was not being picked up by MetaModule.

I also tested merging all the open PRs together, and it's not going to be a problem to merge.

So I'm going ahead with this!