does not remember workgroups correctly

[simonmichael]

When creating a workgroup, it overwrites the previously created workgroup as well. To reproduce:

• create buffer "1"
• create workgroup "1"
• create buffer "2"
• create workgroup "2"
• open workgroup "1"
• expected: buffer "1" appears. Actual: buffer "2" appears.

This can also be seen in the workgroups file, with the help of #110.

[redguardtoo]

39927b6 clear the session in memory after loading/saving the workgroup (Chen Bin)