Terminal: Add header VTKeyTbl.h

... exporting what is defined in VTKeyTbl.c.
Terminal: Pull user input handling into state classes
BasicTerminalBuffer: Add {Previous,Next}LinePos()
Terminal: Add a hyperlink mode

When holding down Command, text under the mouse is checked whether it
looks like a URL or a local path. If so, it is highlighted and can be
clicked, which will open the URL/file. Right-clicking opens a context
menu with items for opening the link/file or copying it to the
clipboard. When additionally holding down Shift, path prefixes up to
the component under the mouse will be considered (no effect for URLs).

Changes:
* Add HyperLink class. Encapsulates a type, the address, and an
  optional base address. Features an Open() method to open the address.
* Move/add some string constants to TermConst.
* Move TermView::CharClassifier to top level and rename to
  DefaultCharClassifier.
* Introduce TermViewHighlight and TermViewHighlighter. The former
  refers to a range of text in a TermView's text buffer. It also
  contains a pointer to a TermViewHighlighter object, which specifies
  how the text range shall be rendered (colors and attributes).
* TermView:
  - Add respective _{Add,Remove}Highlight() methods and adjust the code
    to support highlights.
  - Make the selection a TermViewHighlight. At least its visual aspect
    is now handled like other highlights.
  - Introduce an inner TextBufferSyncLocker. It is used instead of
    BAutolock when locking the text buffer to synchronize the visual
    buffer with it. After it unlocks it calls
    _VisibleTextBufferChanged(), if the visual text buffer has changed,
    which in turn calls a new callback on the active state.
  - Add WindowActivated() and ModifiersChanged() callbacks to the state
    interface.
  - Add new states HyperLinkState and HyperLinkMenuState which
    implement the new feature.

Fix modifier issues
BURL: Make the output to stderr DEBUG only
TTracker::RefsReceived(): Forward "be:*" message fields

In some cases /bin/open puts some additional fields in the
B_REFS_RECEIVED message, but those were completely ignored. This makes
opening paths with appended line/column numbers actually work as
expected.
Fix Terminal build.