fix: TextBox, NumberBox, PasswordBox Placeholder Not Working Properly.

[textGamex]

Pull request type

Please check the type of change your PR introduces:

• [ ] Update
• [x] Bugfix
• [x] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [x] Documentation content changes

What is the current behavior?

• When Password is set before setting PlaceholderEnabled, both PlaceholderText and Password will appear simultaneously.
• If PlaceholderEnabled is false and PlaceholderText is not empty, clearing Text will still display PlaceholderText.

Issue Number: #1043

What is the new behavior?

• fix this bugs.
• If PlaceholderEnabled is set to false and PlaceholderText is not empty, clearing Text will not display PlaceholderText.

Other information

Added a new dependency property CurrentPlaceholderEnabledProperty.

[textGamex]

I don't understand why the default value of PlaceholderEnabled is true, and when we assign true to it again, both Password and TextBox will be displayed at the same time.

[textGamex]

In fact, when we set PlaceholderEnabled from false to true, if Text is empty, the PlaceholderText will not be displayed immediately. It will only be shown after the OnTextChanged event is triggered. A solution would be to add an OnPlaceholderEnabledChanged event, which calls the SetPlaceholderTextVisibility method when PlaceholderEnabled is set from false to true.

I'm not sure if there is a better solution, or should we put this issue on hold for now, what do you think? @pomianowski