This removes a lot of allocations and speeds up the compiler step
a bit. Sadly, this patch is very invasive as it touches a lot of
files. That said, each individual hunk is pretty trivial.
For a non-trivial real-world example, the impact is significant,
we get rid of ~29% of all allocations and improve the runtime by
about 4.8% (measured until the viewer loop would start).
Before:
```
Benchmark 1: ./target/release/slint-viewer ../slint-perf/app.slint
Time (mean ± σ): 664.2 ms ± 6.7 ms [User: 589.2 ms, System: 74.0 ms]
Range (min … max): 659.0 ms … 682.4 ms 10 runs
allocations: 4886888
temporary allocations: 857508
```
After:
```
Benchmark 1: ./target/release/slint-viewer ../slint-perf/app.slint
Time (mean ± σ): 639.5 ms ± 17.8 ms [User: 556.9 ms, System: 76.2 ms]
Range (min … max): 621.4 ms … 666.5 ms 10 runs
allocations: 3544318
temporary allocations: 495685
```
Preparation for multi-components
Note that this had to rename one instance of TextStyle because it
conflicts with the struct of the same name used in the FontSettings
in the style. This wasn't a problem before because it shares some
property in common, and the the order of processing of component has
changed leading to the other one being generated.
(But that is a wider bug in the compiler outside of the scope of this
refactoring)
Updated the version from 1.1 to 1.2
Renamed the header to "Slint Royalty-free Desktop, Mobile, and Web Applications License"
Added definition of "Mobile Application" and grant of right
Moved "Limitations" to 3rd section and "License Conditions - Attributions" to 2nd section
Added flexibility to choose between showing "MadeWithSlint" as a dialog/splash screen or on a public webpage
Moved the para on copyright notices to section under "Limitations"
