Embryo of developer documentation :-)

This is a document with information I found most useful as a
new contributor to TextMate - basic stuff a new contributor is likely
to get in contact with.

It's of course incomplete, sparse, can be not well worded and even contain
mistakes. But someone had to make the first shot :-)
Show warning if undo will delete non-empty document

Closes #1197
Support text links with VoiceOver

Now links in text (e.g. links to RFCs after installing the Hyperlink Helper bundle)
are announced as such with VoiceOver and it is possible to search for them with
VO-Cmd-L.

TODO:

* launching links with VO-Space is not supported yet, but Enter (Fn-Return)
  does the same thing and is even more reliable.
* link URL is not supported, so it is not possible to learn the URL
  with VO-Shift-U.
Track softline count in layout
Provide conversions between softlines and buffer positions
Use softlines instead of hardlines for accessibility

Fixes #1149.

VoiceOver expects AXLineForIndex and AXRangeForLine to be consistent with
the movement by line using arrow up/down. That means it expects "line"
to mean softline.

VoiceOver also distinguishes commands to move by:
* paragraph (hardline): VO-Shift-PgUp/PgDown
* line (softline): VO-up/down

These are available when interacting with the text area (VO-Shift-down).

This commit therefore fixes the AXLineForIndex and AXRangeForLine
to make "line" mean "softline" instead of "hardline" as was the case
until now.
Fix accessibility visual bounds ending at line boundary

Accessibility visual bounds for a range of a single softline should always
be also visually restricted to the softline. This is in order to be consistent
with implementation in Apple's apps (e.g. TextEdit).
Support getting AXVisibleCharacterRange accessibility attribute