Put breakpoints on the line number margin and let a file type restrict them - #357

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Three changes that belong together, all around the editor margin.

Breakpoints move onto the line numbers: a line that carries one shows the dot
in place of its number, as Rider and VS Code do. MeasureOverride stays
inherited, so the column is exactly as wide as it was without breakpoints.

BreakPointMargin stays and is marked obsolete. Nothing in the tree points at it
any more, but it is public API, so it goes out with the next batch rather than
on its own. It loses a leak on the way: the store outlives the margin, so the
subscription moves into OnTextViewChanged and is released when the margin
detaches.

ExtendedTextEditor handed every margin a new BreakpointStore(), so a
breakpoint lived in an object per editor that no debug session ever saw. The
margin gets BreakpointStore.Instance instead, which also lets a breakpoint
survive closing and reopening its file.

ITypeAssistance gains BreakPointLinePattern, a regular expression whose null
means no restriction, so a language without a rule notices nothing. Not every
line of every language can carry a breakpoint: in assembly one on a data word
does not fail but slips, because the line table has no entry for it and the
backend moves it to the next line that has code. The rule travels as data and
not as a predicate - a callback would put the core into plugin code on every
click and every pointer move over the margin. Removing a breakpoint always
stays possible, and the preview dot appears only where a click would take
effect.

SetEnableBreakpoints therefore takes the ITypeAssistance instead of a bool:
the margin needs the flag and the pattern as a unit.

BreakPoint carries whether the target armed it, and the margin draws a grey
hollow ring when it did not, inside the same diameter so the column keeps its
width. The store raises VerificationChanged for that, because the change
alters no collection and a margin listening only to CollectionChanged would
never repaint. The store also carries whether the target is running; while it
is, the margin takes no breakpoint in either direction and says why. Both stay
inert until a debug session sets them.