Revise 02: Shared Concepts in MEI

[kepper]

This issue can be used to discuss chapter 2 of the guidelines.

[Grabitz]

I spotted the following inconsistencies:

• 2.2.3 shared concepts:@stemis the only attribute without an explanation in the guidelines.
• 2.3. Common Attributes: @xml---base in guidelines but @xml:base in specs, @xml---id in guidelines but @xml:id in specs
• 2.4.2.1, 2.4.2.2., 2.4.2.3. some pictures are not displayed (I see question marks instead)
• 2.4.2.3. I can't find @glyphname and @glyphnum in specs
• 2.4.3.3. I can't find @clef.line in specs
• 2.4.4. @lstartsym @lendsym @lstartsymsize @lendsymsize in guidelines, but all of them are in specs without the front l
• 2.4.5. I can't find @scale as an attribute of in specs