Added public functions for button label and button enable/disable.

[ihakov2]

The purpose of this PR is
1. To reuse GFX_button object for On/Off Start/Stop operations
to save memory and convenience.
2. Set Enable/Disable button to check for click and process or skip (ignore) 'button click' checking and processing.
When the button is disabled it will keep currstate of the button as false
No limitation.

Test:
void loop() {
startButton.drawButton(true);//simulated click
delay(400);

if(started) {
startButton.setLabel("Stop");
configButton.setEnabled(false, ILI9341_LIGHTGREY);
}else { // not started
startButton.setLabel("Start");
configButton.setEnabled(true, ILI9341_BLUE);
}
Serial.print(F("\nstartButtonLabel="));Serial.println(startButton.getLabel());
Serial.println(configButton.isEnabled()? "configButton is Enabled":"configButton is Disabled");
started=!started;
delay(2000);
}

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