Improve tag-goto popup

Improve tag-goto popup

1. Show signature for all tag types using get_symbol_name() when
get_symbol_tooltip() returns NULL. Modify get_symbol_name() to drop
line number when needed. (More or less taken over the complete
implementation from Colomban)

2. Add scope information to the signature both when using get_symbol_name()
and get_symbol_tooltip(). (Based on Nick's idea)

3. Truncate the length of the popup to at most 80 characters (More or less
taken over the complete implementation from Nick)

4. Improve formatting of entries in the popup so individual pieces of
information are easier to distinguish:
- file name and line number are always in italics
- the following signature is in small monospaced font

5. Add tooltip to every entry (based on Nick's implementation).
- split the tooltip into two lines - the first line shows the file and
the line number, the second line the signature
- both of the lines are formatted in the same way as described in (4)

(Thanks to Nick Treleaven and Colomban Wendling for the original
implementation.)