WiFiManagerParameter max length and encapsulation

[dmadison]

This PR performs a few fixes around the WiFiManagerParameter class to improve clarity, usability, and encapsulation:

• The length parameter has been changed to maxLength for all class functions
  • The parameter refers to the max length of the form and the size of the internal buffer, not the length of the string passed as an argument. The internal name (_length) has not been changed to maintain compatibility with user code, as it's exposed as a protected value and through friendship with WiFiManager.
• setValue() now has a version without a max length argument
  • The original 'length' argument was confusing as a user, since it seemed to refer to the length of the string passed in while it's actually the max length of the buffer / value in the form. This new function allows for changing the value without changing the max length.
• getValueLength() has been changed to getValueMaxLength()
  • This better represents what is being returned, since it's the maximum length of the form / buffer and not the length of the value (i.e. what's currently in the buffer, and what's returned from the webpage). The original function is marked deprecated and has not been changed for compatibility reasons, but it would be nice if it returned the length of the value in the next major release.
• Friendship between WiFiManagerParameter and WiFiManager has been removed
  • The few internal calls in WiFiManager have been changed to use the public functions and there is no remaining reason for the friendship to exist. Removing the friendship boosts encapsulation.
• setValue() now checks for a negative max length
  • All of the corresponding 'length' arguments should probably be unsigned (size_t). Again, this was not changed so-as not to break backwards-compatibility.

This PR should have no breaking changes. The few items marked deprecated should be removed on the next major release.

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If this is merged, here are the changes to make on the next major release:

• WiFiManagerParameter::_length should be refactored to _maxLength
• WiFiManagerParameter::getValueLength() const should be removed
• All WiFiManagerParameter maxLength arguments should use size_t instead of int

[dmadison]

Closing this in anticipation of further changes.

[tablatronix]

Ok please do not PR too much in one issue, I wont merge it