Check valid and safe html markup in tooltip text

[eharris]

As discussed in airsonic/airsonic#1154 I think we need to continue to support some form of formatting within tooltip strings from the internationalization properties files.

With an eye toward making using standard html markup as "safe" as possible, it would be nice to have some validation that the tags within the strings are valid, and also restricting which tags can be used.

I believe a reasonable set of tags to allow (somewhat in order of importance):

• <br> - Line break
• <b><strong> - Bold text
• <i><em> - Italic text
• <p> - Paragraphs
• <ol><li> - Ordered lists
• <ul><li> - Unordered lists

Others that aren't really necessary but might be nice:

• <sub> - Subscripted text
• <sup> - Superscripted text
• <small> - Smaller text
• <mark> - Marked/highlighted text
• <del> - Deleted/strikethrough text
• <ins> - Inserted text

Tags that should definitely not be allowed (in my opinion):

• <a> - Links
• <img> - Images
• Any other tags that could cause other resources to be followed or loaded