Splash screen version check should use md5 hash instead of file modification date

[elexis]

As pointed out in #4391, a file modification date isn't a good identifier to track content changes. The splash screen shouldn't be a bad example and check for actual content changes by using a quick md5 hash of the contents to compare.

​https://github.com/elexis1/0ad/commits/splashscreen ​https://github.com/0ad/0ad/compare/master...elexis1:splashscreen.diff

Also includes the rephrasing mentioned in comment:6:ticket:4391, a GUI element size fix, duplication removal, moving of code from XML to JS and saves splashscreenversion when opening the page (not when closing it via OK).

[elexis]

Should the welcome screen be considered read when showing it or when clicking okay?

[elexis]

​Phab:D1575 for the md5sum version check of the lobby terms.

[elexis]

In 21850:

Persist the lobby Terms Of Use and Terms Of Service checkbox if the logged in user and the accepted versions of the pages didn't change since last login, refs #5218.

This way the user is only forced to read the Terms again that changed or if the user logged in from a different machine.
Use md5sum since it is sufficiently resistant against collisions and doesn't freeze the window for 2 seconds like EncryptPassword / SHA256 does, refs #4399.
Use 0 instead of empty string in default.cfg, refs #3990.

Differential Revision: ​https://code.wildfiregames.com/D1575
Partial review by: Vladislav

[Imarok]

Move tickets to Misc. UI as "UI & Simulation" got some sub components.

[Stan]

Unlikely to happen for A25

[Stan]

Unlikely to get done for A26

[Freagarach]

Pushing back.