improves the readability of the code that handles the mouse

mouse actions with alt key have a bad effect, alt hides/shows the menu
I added a little workaround to fix this bad effect:
I cancel the alt key press when a mouse action occurs
adds zoom factor in status bar
settings edition no longer modifies the zoom factor
adds new hotkeys :
"Zoom 100%" default CTRL+0 NUM KEYPAD
"Zoom increment" default CTRL++ NUM KEYPAD
"Zoom decrement" default CTRL+- NUM KEYPAD
help update
1.07.0 beta 6 (26 May 2013)