Implements timestamp_t structure

[jbrazio]

As a followup of #4298 (comment) this PR implements a new timestamp_t structure which allows easy elapsed time manipulation. When compared with the previous code having #define PRINTCOUNTER enabled, it saves 72 bytes of binary size.

[petrzjunior]

What about doing something similar in LCD Info menu (#4312)? I did the format as
dddddd hhh mmm with two spaces before to fit exactly to 16x2.

[jbrazio]

Without changes you may use the hour() and minute() to reuse the calculations but~~~ we can do better by having another string formats, replace toString() with another name.~~

[petrzjunior]

Something like... toStringYears() and toStringDays()?

[thinkyhead]

Hmm, the compiler seems to be doing smart things, using registers a lot and so forth. The inline keyword makes no difference. I can't get the compiler to be more optimal. So… merging forthwith.

[thinkyhead]

Merged!