Visualize data segment

[jeissonh]

Visualise data sections of the user program as a data segment:

• [x] Extract global variables with ctags during building process: #143
• [x] When breakpoint at entry point is reached, create GDB's variable objects for each global variable
• [x] Draw each global variable in memory rows of DataSegment
• [ ] If more user global variables than memory rows, generate segment overflow for the unit
• [ ] Update global variable values during program execution
• [x] Clean data segment variables when stopping visualization