Maintain datasheets for TTL chips and rename the TTL entry

[gtxzsxxk]

In these days, the 74** series chips have been no place in the market, because their speeds are too low. The 74hc series have take their place, however the names in logisims are still 74xx. Meanwhile, logisim-evolution seems to provide few information about the chips in the library, which in turn led the users to find the datasheets themselves, especially the name is 74** rather than 74hc** or 74hct** or 74ls**, which makes it more hard to seek for the chip information. So given the two problems above, I suggest

• rename the entry TTL to 74 logic family. Technically speaking, some “TTL chips" we can buy in the market have already been CMOS chips. If you try buying 74** chips, you can only find 74hc** and other CMOS products in stock.

• we maintain the datasheets and put it under the regions, where the users once choose the TTL chip could reach, for example, add a button in the property menu. And in fact we don't need to maintain the PDF files because It may cause some legal or intellectual-property problems. We can simply put a URL there, once the button clicked, the browser would show the desired datasheet for the users. So we need to find the reachable datasheet URL for each 74 logic family chip, and ask the contributors to provide the URL for any coming 74 chips.

Do you think my plan applicable? If OK, please assign this to me. I would do some research on alldatasheet's policy about whether we could refer to their PDFs.

[BFH-ktt1]

@gtxzsxxk : I like the idea and if it is useful for our users I'm very happy to review any PR in this direction. Please note, however, that the mainstream developers are not actively developing any more this library, as we have no use in our courses for this library.