Properly initialize Scala editor (quick fix/interactive error reporting works even before a save).

Compilation units in the JDT are backed up by a `buffer`, holding the source that 
hasn't been saved yet. We use the buffer to get notification of character changes,
and in turn notify the presentation compiler. Without a proper buffer, errors (and
quick fixes) are not reported either.

Buffers are created lazily by the JDT, and we mimic the JDT code in 
`buildStructure`.

Fixed #1001094, Fixed #1001337.
Make the class path validator regexp stricter.

This protects against jars that might have a 2.x.y version number that looks like
a Scala version, but it's not. We check now that the third digit, if exists, is a
digit, not any character. (previously it would fail the aspects_2.1.0.nightly jar)
Merge pull request #291 from dragos/urgent/fix-classpath-validator

Make the class path validator regexp stricter.
Merge pull request #287 from dragos/issue/no-quick-fix-1001094

Properly initialize Scala editor (quick fix/interactive error reporting ...