Task highlighting

[fanwgwg]

• Highlight the task(s) just added/marked/unmarked/renamed/scheduled Enhancement: Highlight tasks Jim just updated in the UI #160
• Highlight existing tasks that already exist when adding some duplicates
• Add short key ctrl+z for undo Hitting Control/Command+Z at any time while in the app should perform an undo command #181
• Show completed time for completed tasks Show completed date/time for tasks in the done column #184
• Remove the placeholder of time for floating tasks Text should be vertically centralised for tasks without  #183
• Change the shortcut key of opening of help window from F5 to ctrl+H

[fanwgwg]

Gui tests fail, going to investigate on it.

[burnflare]

@fanwgwg Mind attaching screenshots here of how the new elements look like?

[fanwgwg]

This is how it looks like after the changes. @burnflare Is this fine?

[burnflare]

Ooo nice, variable cell heights. If I have a title that's 2 lines long will it be taller too?

[fanwgwg]

Yup it will resize automatically!

[rachx]

Actually, it is showing the last updated time instead of the completion time.
So if you rename a completed task, the time will changed as well >.<
I still think it will be useful to show the time of events/deadlines that have passed

[rachx]

Fixes #117, #181, #183, #184
Revise #160 after update-model

[burnflare]

@rachx @fanwgwg Is this good to go in?