[FEATURE] Memory functionality

[tommens]

Describe the new functionality you would like to see

• The calculator should allow the user to store in memory a list of favourite expressions that he would like to reuse later. The size of this memory can be finite, but should be configurable by the user through some option in the application's interface.
• The calculator should have a history log in internal memory of all expressions that have been previously entered and evaluated. This log should be easily accessible, for example to select and reuse expressions or evaluated results that have been used somewhere in the past.
• Given the presence of such a history log, it should be possible to have multiple undo/redo functionality. (E.g. the user should be able to go back into the entire history stored in the calculator's memory.)
• It should be possible to save the history (of all favourite expressions and of the history log) in a human-readable text file. It should also be possible, when closing the calculator application, to still have access to its recent history when opening the application again (e.g. by auto-saving the history in a file before closing the application, and reading the file when opening the application). It should be possible to open any history file that was previously saved.

Illustration

• A partial, incomplete example of the intended functionality is shown in the bottom part of the figure below: one can see the history, and can use a drop-down box or arrows to select elements from the history