Add support for PROGMEM-based text strings

[ImpulseAdventure]

Feature request: add support for read-only text strings located in PROGMEM (flash)

• Enabling such a method would support optimized SRAM usage on Arduino
• User calls with PSTR(). ElemCreateTxt() / ElemCreate() may require typecasting param with (const char *)
• DrvDrawTxt() could use pgm_read_byte()
• Consider creating enum (GSLC_TXT_PROGMEM with negative value) in place of buffer length to flag the string as requiring PROGMEM access versus SRAM. Change nStrBufMax params to be int16_t instead of uint16_t.

Example:

  static const char mstr[] PROGMEM = "Count:";
  pElem = gslc_ElemCreateTxt(&m_gui,GSLC_ID_AUTO,E_PG_MAIN,(gslc_tsRect){20,60,50,10},
    (const char*)mstr,GSLC_TXT_PROGMEM,E_FONT_TXT);

[ImpulseAdventure]

This feature has been added to GUIslice v0.8.1. From user code perspective only real change is extra call to gslc_ElemSetTxtMem().