Add directory support to fs_driver

[ThomasFarstrike]

Support directory operations (dir_open_cb, dir_read_cb, dir_close_cb) in fs_driver.py so that the file_explorer widget, when using fs_driver, is able to browse the filesystem.

I validated this on the unix and esp32 targets with:

import fs_driver
fs_drv = lv.fs_drv_t()
fs_driver.fs_register(fs_drv, 'M')
file_explorer = lv.file_explorer(lv.screen_active())
file_explorer.explorer_open_dir('M:/')
def file_explorer_event_cb(event):
    path = file_explorer.explorer_get_current_path()
    file = file_explorer.explorer_get_selected_file_name()
    print(f"Selected: {path}{file}")
file_explorer.add_event_cb(file_explorer_event_cb, lv.EVENT.VALUE_CHANGED, None)

If you get a UnicodeError, that's not due to this code but the binding code generator and how it handles char * vs void * arguments. See this discussion.

[ThomasFarstrike]

With that additional commit, the UnicodeError is fixed now. Ty!

[kdschlosser]

And it was able to be done without having to make any changes needing to be made in LVGL...

Thanks for spending the time to make the PR. I always want people to get the credit for the work they do and PR's is the way that gets done.

If you are up to the challenge I would love to have some help with writing a new script to handle the binding code generation. The existing script is a mess and hard to understand what is happening. I also want to simplify things by flattening the API so it matches LVGL's C API. This will make it easier for people to use. A user will be able to select which API style they want to use at compile time. I will deprecate the current API and have a warning shown at compile time if it is used. That way it gives people some time to transition to the LVGL API.

The code generator script will be vastly easier if the API gets changed. I also want to write it in a manner that makes it easy to understand what is happening. create an actual module with separate source files and not 4K lines of code dumped into a single file.

If you are up for it lemme know. I will create a new repository where we can work on it and I will give you write permissions to that repository so we can both add to it. It will take a while to get it done.