[LVGL] Report error when mandatory build directive is missing

[mvladic]

Report error when mandatory build directive, for example //${eez-studio LVGL_NATIVE_VARS_TABLE_DEF}, is not found in any of the file templates.

This could happen if project was created using older version of Studio when specific directive wasn't mandatory. Or when project is created without flow support and checkbox Flow support is selected. For the later case we can also ask user if he whishes to change all the file templates as if project is created with the flow support in the beginning.