[3.4.x] Some form elements no longer have their context menu buttons

[Sa'ar Zac Elias]

• Open a sample.
• Add a image button.
• Right click on it.

Notice that the "Image Button" option is missing.

The same happens for textarea, text input, radio button and checkbox.

[Sa'ar Zac Elias]

We should check if the input is readonly by request or it became readonly by our code to prevent it's editing.

[Frederico Caldeira Knabben]

The change makes a lot of sense.

Before committing, please take the following in consideration:

• Let's just make the fix more generic, so it can be used for any need. Instead of calling the marker "_cke_lockedInput", let's name it "cke_notReadOnly".

• Also, usually for such flags, we use "1", instead of "true", to save 3 bytes of code size :)

[Sa'ar Zac Elias]

Fixed with [5781].