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

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