sequence diagram: Add messageTextBox and loopTextBox styles

[matevz]

Proposal

The message text is sometimes difficult to read, if put on top of the arrow. For example in the official docs notice the loop titles and messages overlapping with the arrow:

This is especially annoying, when the arrows are darker. I suggest we add a box behind the text which is transparent by default, but could be adjusted when needed.

Colors

.messageTestBox and .loopTextBox classes with transparent background by default.

Screenshots

No response