* Convert theme select, theme edit and color edit pages to LVGL flex.
Add 'Edit Color' string to language files (requires translations).
Simplify theme edit 'onCancel' logic.
Fix size of buttons in ConfirmDialog for portrait layout.
Truncate item strings in ListBox to fit available space.
Set window background color on Page and TabsGroup classes instead of using 'paint' function.
Replace strncpy and strncat with strAppend - avoid possible unterminated strings and string overrun.
Truncate display of long name and author strings in theme pages.
* Revert code to truncate ListBox entry strings as it causes other issues.
* Adjust layout for theme color editor to fit large hex string size.
* Remove unnecessary copying of the ThemeFile object being edited.
Refresh TabsGroup page background color when theme changes.
* Set selected theme in list on long press as well as short press.
Comment out unused 'title' in ListBox.
* Handle default theme like custom themes on SD card:
- show logo when selecting theme
- support background_WxH.png names for background images
Use f_stat( via isFileAvailable) instead of opening and closing files to check for existence.
* Remove debug print.
* Fix rebase.
