Ability to render `html` (or some rich content) in balloon.

I wanted to be able to control the font that is shown for a particular tooltip balloon in MacVim. The use case is for showing compiler errors in monospaced font (otherwise, they render terribly). Then, I quickly realized that I only want some of the tooltip to be monospace and some to be rendered with standard text font. I also wanted greater control over the content in general, and realized that the best way to do this would be to allow the tooltip content to accept arbitrary HTML and have that be rendered using WebKit. I believe this could be done entirely in MacVim and not concern Vim itself. When writing a plugin that shows tooltips, you would want to check if you're running inside of MacVim to determine if you should show rich HTML content vs plain text.

Inizia dall’implementazione dei tooltip o dei balloon di MacVim e segui come viene attualmente eseguito il rendering dei contenuti forniti dai plugin. Verifica il confine WebKit proposto e come debba essere selezionato HTML avanzato rispetto al testo semplice. Il lavoro è completato quando i plugin possono richiedere contenuti avanzati per i tooltip in MacVim, mentre i tooltip esistenti in testo semplice continuano a essere supportati; non vengono indicati file o test.