Hi

This bug is to fix 4614, 4619 and 4870.
Here is what it does:
- Change the top confuse text "current program counter" to segment
combox box. Users can choose the segment from the combo box to display
and adjust the offset by spin button. Some segment is too long to
display, it's be handled the by the following rule.
- If the specified startAddress is too smaller that the lastKnownFrom
value, and the UI can't handle some many data, it would be set to
display about 50 instructions at disassembly window and 20 rows at
memory window from the startAddress. 
- It would be handled by the same way if the user only changed the
endAddress.
- Add warning dialogs when the address can't be accessed and when the
symbol is not in current context. The warning messages are as same as
these of gdb.