fix: resolve caption state handling and CSS issues

[idebenone]

Problem

• The caption feature in the Image Tool wasn't properly reflecting the correct state when preloaded data contained a caption.
  
• The tune state for the caption wasn't consistent.
  
• CSS styling for visually showing/hiding caption field was altering the inline-toolbar position.
  
• select an image button was hidden in uploading state causing a bad UX if user doesn't select any file.
  

Solution

• Corrected the logic to properly handle the caption state when loading pre-existing data.

    if (data.caption) {
      this.setTune('caption', true);
    }

• Introduced an utility function which determines the tune state directly reading the CSS rather than depending on this._data values.
• Instead of depending on display property, relying on visibility fixed the problem.
• Removed button's styling display: none during uploading state.

[neSpecc]

why we depend on style instead of actual data state?

[idebenone]

The reason why I considered style because the way caption in features needs to be interpreted as boolean while its a string, leading to the inconsistency in maintaining the tune state (On successive enable/disable while resetting the caption). This doesn't feel like a a permanent fix.

So I took a step back and thought of introducing a withCaption field which is of type boolean like other tune values which are very consistent with tune state.

I also tried to turn the field caption into an object:

{
    state: boolean,
    value: string,
}

But introducing a complex structure compared to a straight forward boolean seems unnecessary.

[neSpecc]

I think it's redundant property since we already have "caption" field that can be empty.

Developer can disable caption for all image blocks using tool config. It should not affect output data