[Feature Request] Option to offset Clocker's time interval

[n0shake]

Original request: #122

This will likely mean we'll need to support offsetting the time in the following sub-systems:

• Panel
• Menubar
• Time Slider (consider both future and past times)
• Upcoming events view
• Upcoming event in the menubar
• Reminders

Other considerations:

• It'll be good to provide a small UI indicator to let users know that the offset is in place, lest they be confused and miss an important event due to this feature.
• Include if this feature was enabled in bug reports
• Decide where to place the offset preference
• Decide limits for offset (positive, negative?)