Fixed no boot on TFT GD variants + cleanup

[digant73]

BUGFIXES:

• Fixed no boot for TFT GD variants reported on [BUG] BTT TFT35 E3 v3.0.1 #2884: bug introduced on PR cumulative patches + cleanup #2873. All the TFT GD version are affected, so this bugfix could be considered major for some users.
  Bug was in os_timer.c. In particular:
  TIMER_INTF(TIMER6) &= ~TIMER_INTF_UPIF; // clear interrupt flag
  was wrongly set to (see missing ~):
  TIMER_INTF(TIMER6) &= TIMER_INTF_UPIF; // clear interrupt flag
• Fixed random freeze using Menu->Settings->Connection->Disconnect button: bug was due to unallocated memory access. That bug was wrongly reported as already fixed by PR Serial bugfix on init/deinit #2822
• Fixed missing settings initialization before loading config.ini: as reported in [BUG] BTT TFT35 E3 v3.0.1 #2884 the bug caused a random value for Listening Mode setting not provided in config.ini. In general, a missing setting value in config.ini caused a random value for the setting

IMPROVEMENTS:

• Cumulative patches: it includes fixes/improvements provided on Numpad bugfixes #2856 and Icons update from USB bugfix #2879 (they seem non being merged) plus some cleanup. Affected files are boot.h/c and Numpad.h/c. All credits to kisslorand
• Cleanup on TouchProcess API: API renamed to Touch_Screen and moved from TFT\src\User\API\UI to TFT\src\User\API folder (that API is not a UI API). Furthermore, functions renamed appending prefix TS_ (common naming convention used for other core APIs)
• Folder of MKS TFT28 bootloaders properly organized: original and patched bootloaders are provided in separate folders
• Minor cleanup: Moved definitions from .h to .c on some files (e.g. config, boot API) etc...

NOTE: PR originally planned to be released after the merge of the remaining pending PRs (buzzer based on Interrupt, bugfix on Numpad and fw update from USB stick) as a final cleanup. Just anticipated to fix the issue on missing boot on TFT GD variants.

fixes #2884

PR STATE: Ready for merge