a date is a date, without time

And use DateTime instead of manually concatenating Date and Time.

Change-Id: I6f9fb988beea2a9a17c976d3b98860c4f9ed063b
more flexible handling of current date/time hotkeys

Let Ctrl+; current date and Shift+Ctrl+; (Ctrl+:) current time behave more
flexible, depending on the formatting (and as such the type) of the existing
cell content.

key date on time  cell => current date + time of cell => date+time formatted cell
key date on other cell => current date => date formatted cell
key time on date  cell => date of cell + current time => date+time formatted cell
key time on other cell => current time => time formatted cell

Change-Id: I2ccbfbab3c50c36a72c4db4696520e32f94caefa