Mouse wheel zoom and keyboard modifiers not working

[tudorp]

In latest next branch snapshot the mouse wheel:

• only scrolls through the signals vertically when the window height is not fitting all the signals (even when using SHIFT modifier, EDIT: this is actually a duplicate of #163 ) and no zoom when pressing CTRL
• only scrolls left/right if signals fit in the window (no zoom even if the setting is enabled through Preferences dialog, nor when CTRL modifier is pressed)

In version 0.9.8 the CTRL modifier was working, but the SHIFT didn't seem to be implemented