Nice program. I just have a few suggestions though and they are really just preference by me and not more functional one way or the other. I would remove the tearoff function of the drop down menus. I know for me I never use it and it just looks ugly with that line at the top of every drop down.

Second I would not have the numbers dissapear before the next number is entered during a calculation. This could be achieved by maybe using a flag variable and set the flag when a calculation key is pressed and then when entering text check for the flag and if it has a value or a cetrtain value then clear before entering the text and if not then just append the text onto the existing number.

Lastly there seems to be an error when using decimal points in calculations. It only allows 2 decimal places after the decimal point and it does not calculate sometimes.

Other than those, good job!