generalize lv-font creation

In https://github.com/InfiniTimeOrg/InfiniTime/pull/1097 new font
generation capabilites were added. Generalize the font creation to
make it possible to reuse the `displayapp/fonts/CMakeLists.txt` file
for `InfiniSim` and just add the new cmake file to the project and
link against the new `infinitime_fonts` target.

In the following a list of changes.

Allow non-global installed `lv_font_conv` executable installed with

```sh
npm install lv_font_conv@1.5.2
```

In CMake we search for `lv_font_conv` executable. Add the found
executable to the python script `generate.py`, to remove the need for
`lv_font_conv` to be in the path.

Search for `python3` executable, if CMake version 3.12 is available.
Otherwise use `python` as hard coded executable.

Instead of adding the generated fonts to `SOURCE_FILES` variable, create
a static library `infinitime_fonts`. Link this library to the
executables instead.

Use `add_custom_target()` together with `add_custom_command()` to
generate the font.c files once (like the original PR does).

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