[LVGL] Allow images from different sources

[mvladic]

We currently only support images coming from variable of type lv_image_dsc_t *. Support also:

• from string (const char *, i.e. "S:/background.bin"), and
• from LV_SYMBOL_...

Consider supporting this with minimal changes in auto generated source files.

[mvladic]

For strings, we could ad an additional property to the BItmap:

[x] In the runtime load image from the string: S:/{name}.bin_______

or even as Project Settings property, so that all your images by default are from strings.

For symbols, you could directly enter for the image source property:

This means that we should forbid bitmap/image names to start with LV_SYMBOL.